According to what I have heard (eg. in relation to our duty to evangelise for the faith) Christians are generally supposed-to be ready to answer any questions by anybody at any time...
For this to be a sensible thing, we Christians need to be reasonably confident that we understand the assumptions behind the question in the same way as does the questioner. But - in the modern Western nations, here and now - most questions about Christianity are not merely based on ignorance, but on falsehood and misrepresentation.
So if somebody asks Are you a Christian? we cannot, and should not, assume that the questioner understands, even at a basic level, what 'Christian' means, or what 'being a Christian' entails.
Therefore, if you simply answer: Yes!; then you have very little idea what that answer means to a typical modern person, except that it almost-certainly does not mean the same to the questioner as it does to the answerer.
You have, in fact, probably thrown-away the opportunity for genuine evangelisation.
Typically, the average modern person will be asking a 'heads I win, tails you lose' question; and will simply have their prejudices reinforced by a Christian returning a simple answer to a simple question. This is counter-productive.
If not, then what?
When trapped by another's assumptions, we simply-must make clear that our own assumptions are different - that is far more important than a confident, snappy riposte.
Modern Christians have been locked-into a category by mainstream social and professional discourse - therefore we cannot participate in such discourse. We must disrupt that discourse by our response; we must Not simply tick the box or follow the flow-chart...
This Does Not Work.
(Nor is it truly honest.)
What we should do is quite simple - and that is to ask back what that particular person means by their question; and only when we are satisfied that we understand what assumptions lie behind the question, will we answer it.
Something along the lines of "I am happy to answer your questions, but I need to understand more clearly what you are asking me by Are you a Christian, Do you believe in God, Do you believe in life after death, Do you believe in miracles...
Doing this need not take long, a few sentences will suffice assuming that the questioner is sincere. And if the questioner is not sincere, then that fact should come out pretty quickly too; then we can disengage.
Because the above is what Jesus himself often does in the gospels!
When he is asked the wrong question, or the question is a trap, or he gets asked the same question more than once, or people show that they will not believe him whatever he says - then Jesus disengages - he explains what is happening, or moves away, or says nothing...
In your face and unasked-for impersonal preaching, pat answers, prepared answers, standard answers... all these are now useless to evangelists; since modern culture has pre-immunised the population against them - and they are exactly what fulfils the negative stereotypes.
What exactly to do depends on the specific person and the specific circumstances - and if these can be judged well and we answer honestly according to how we understand things here and now and as prompted by intuition and the Holy Ghost... well, then we might actually do some Good.