It seems to me that above-replacement fertility is a necessity for any valid religion; or, to put it another way, any religion in which the adherents (as a group) voluntarily average less than two children per woman, is of necessity false and evil in some very fundamental way.
The test is necessary but not, of course, sufficient. While sub-replacement fertility is necessarily bad, and evidence of some underlying and over-powering falsity in belief or practice; above replacement fertility is certainly not evidence of a group being on-the-whole-good.
Indeed most groups reasons for high fertility are bad, simply because above-replacement is the natural (biological) state for humans, and humans are sinful creatures who need to be saved.
So, we can apply a filter to Christian denominations around the world and within societies.
All of those denominations in which adherents average less than two children per woman are bad, false and should be rejected as being (in some way, which may or may not be specifically identifiable) on-the-whole evil (their good qualities being overwhelmed by one or more bad qualities).
The bad-ness of low-fertility can conveniently be seen in the age structure of adherents - high median age, proportionately few children and few adults of child-bearing age, and a distorted sex ratio (in other words, not enough men).
And the more extreme, and sustained, is chosen sub-fertility; the worse the denomination, the more anti-Christian.
I am saying that sustained and self-chosen sub-fertility in a group is evidence of such profound spiritual malaise, that it provides an objective measure of spiritual malaise.
In a country like England, this means that all the major self-styled 'Christian' denominations can be revealed as sub-fertile and trending worse, hence net-evil, hence not truly Christian, hence ruled-out of consideration.
Among those smaller, more-specific Christian groups with above-replacement fertility, we cannot assume that any are valid - since we cannot assume that there are any valid Christian groups in England.
It is possible that there are only scattered valid Christians who may be spread thinly across denominations, or outwith organized churches - or perhaps there are simply too few real Christians to register.
But applying this test would suggest where, among which denominations, real Christians might find it worth looking for a church that is potentially good-on-the-whole. As well as providing a quick and simple way of detecting and rejecting pseudo-Christian Churches.
NOTE: To make clear - as a Mere Christian, I believe that real Christians are scattered (but not evenly) across many denominations and are to be found in many congregations - even those of the most corrupt pseudo-Christian churches. However, such individuals would necessarily find themselves at-odds-with their denominations and congregations, and swimming-against the tide of change.