(Note. In order to keep the following concise and focused - I need to assume familiarity with the meanings/ definitions of my basic terminology; and also to draw inferences from a great deal of evidence that I have surveyed. This may require the reader to look at my book The Genius Famine - or at least to browse relevant sections of the Intelligence, Personality and Genius blog.)
Major geniuses are rare, and may even be almost extinct - but some degree of creative genius is much commoner. Creative genius is susceptible of degrees - from a world historical genius to geniuses across smaller domains and with more localized scope. How could such individuals be found?
1. IQ - General Intelligence
Genius requires a very high intelligence. However, a large majority of those with very high intelligence are not geniuses - they are un-creative.
So although a measure of intelligence is a useful first step in finding a genius - it is only the first step.
To be a creative genius requires a high intelligence, and this can be measured relatively and comparatively by IQ tests. The objective level of intelligence thus depends on a proper sample from the comparison population being studied - which varies among races, classes, sexes; and through history.
In the Anglosphere in the middle 20th century it seems that an IQ in the top 5% (125) or so was about where noticeable genius began - since intelligence has declined since then, it may be that a higher IQ is needed nowadays? However, the validity of IQ testing has also declined (i.e. the test does not measure 'g' so precisely as if did) so perhaps the 'threshold' is not necessary different now - but the proportion of geniuses within all categories of IQ will surely have declined considerably.
What about those with 'ultra high' intelligence? At this level, it is not possible to use comparative measures such as IQ - except among young children and using the 'ratio' method of calculating IQ. Thus an 8 year old child scoring in an IQ test at the average level of 16 year old children, can be regarded as having an IQ of 200 (16/8 X 100).
However, childhood IQ is highly but imperfectly correlated with adult IQ, due to differences in development rate (especially between races, classes and sexes). So, a sample of children with high IQ defined in early childhood, will always have lower average IQ as adults.
Adult IQ tests rank test performance and project it onto a normal distribution curve derived from a (supposedly) random sample of the index population - but sufficiently-large and random samples are in practice impossible; so there are never enough ultra high IQ people in a random sample to provide precise IQ measures.
The ultra high IQ societies have developed tests that can differentially rank those who are within the top one or two percent of the population for 'g'; however these values cannot with both precision and validity be expressed in terms of more average IQs (because people with around-average IQs cannot do ultra high IQ tests at all!).
Furthermore, it is found that the concept of 'g' breaks-down among the most highly intelligent - and intelligence becomes much more specific and less general. For instance; the most mathematically/ geometrically intelligent are not also the most verbally intelligent - and the correlation between sub-types of intelligence gets less as intelligence increases. Therefore the precise IQ score at high scores becomes very dependent on the balance of (for example) quantitative and verbal questions in the specific test being used.
In sum: 'high intelligence' is necessary but not sufficient for genius; but it is not possible to provide any precise threshold where 'sufficiently high intelligence' starts.
2. High Psychoticism
Most very highly intelligent people are not creative.
But those who are may be indicated by the Eysenck personality trait of Psychoticism; which is correlated with a wide range of measures of creativity - although trait Psychoticism is mostly negatively defined in terms of psychotic (insane), psychopathic, impulsive and selfish behaviours.
Self-rating measures of Psychoticism depend on honesty, and can be faked. Therefore behavioural correlates of high Psychoticism are more reliable measures - esepcailly as high Psychoticism tends to be 'punished' by social mechanisms.
Most of the most-genius individuals have demonstrated clear evidence of moderate-high Psychoticism in their biography. These can be seen as the opposite to conscientiousness - because high psychoticism tends to sabotage attempts to be conscientious - to follow instructions, to conform to social norms, to work hard and long as externally-imposed tasks etc.
Therefore among the most intelligent people; the most creative are very likely to be less 'successful' then the uncreative conscientious conformists. For example, a person of known high IQ who performed relatively badly in exams - an under-achiever; with results below the level predicted by his IQ; is much more likely to be creative than an over-achiever who performed above the level predicted by IQ.
(Because exam results are predicted by a combination of IQ and conscientiousness.)
(An example is Christopher Michael Langan, who has been claimed to be the highest IQ person alive, and is certainly among the small minority with ultra-high IQ. Langan's biography demonstrates a high Psychoticism personality type (non-conformist, unwilling to accepts instruction or authority, unsuccessful in social terms) - and in a negative sense this is compatible with creativity. So far, with very high IQ and high Psychoticism; Langan would be potentially a creative genius...)
3. Endogenous personality
If high Psychoticism negatively defines a high -IQ group whose non-conformism makes them potentially geniuses; Endogeneity is the positive trait within Psychoticism that leads to actual genius.
Endogenous means internally-generated, and it refers to a personality type that is dominated by inner qualities (and is relatively immune to outer qualities.
At this point it is necessary to go beyond science into metaphysics; because the positive inner qualities that lead to genius need to be dominated by the Real Self, which is also the divine within Man.
It is this being driven by the Real and Divine Self, that is what enables the genius to be genuinely creative. Because to be creative is to be generative of new truth, novelty that is true - and not merely to combine and extrapolate from memorized known truths.
Also; because the genius is in-touch with his Real and Divine Self; it is unlikely that the genius will be an atheist - he may be a deist (acknowledging an abstract deity or spiritual tendency), a theist (believing in a personal God), or may be an adherent of a religion (albeit likely to be unorthodox or heretical, due to the nature of his personality).
The total package
But to be a genius means that this internal drive is also focused and sustained. So an Endogenous personality with show biographical evidence of having been intensely interested for sustained periods (some years, typically), by some internally-driven (not socially normal) subject.
The not-normal, not-socially-imposed nature of the subject is evidence of internal drive; that the drive is intense and sustained is what is required for genius level achievement.
(To return to Chris Langan; his intellectual interests are unfashionable, have been pursued over extended periods, and he is explicitly a theist. I would conclude that by this schema, Langan is therefore likely to be a creative genius.)
There may, or may not, be accessible evidence of achievement by the genius. If the genius has expressed his contribution in writing, making, depicting - these are more likely to have survived; but may have been ignored or rejected - and they may be difficult to evaluate by those of sufficient ability who are not in the same field, or who will not devote sufficient effort and attention.
After all, the expression of genius concepts is a secondary matter from the ability to originate them. And it is possible that the output of genius may be ephemeral - such as conversation.
So, it is apparently possible to find and identify a probable-genius by going-through these stages of evaluating IQ, Psychoticism and Endogenous personality.
What to 'do with' a genius when you find him is another matter!
Perhaps the most valuable thing would be to support them in what they are trying to do - assuming they are not an evil genius! But you could only know someone was evil in motivation if you yourself were already on the side of God, creation and The Good.