Which observation would be the best evidence for a recent founder event or population bottleneck?

The best evidence of such phenomena could be a population's genetic variation decrease.

When a population bottleneck occur it means that for some reason (i. e. environmental events) the size of a population is largely reduced. This leads to reduction of the genetic variation in that new smaller population. When a bottleneck occur, the founder effect is normally seen, which is that one population is established by a small group of a previously larger population leading once again the genetic variation, now though with a bigger population, to be very reduced.