How do we know that a species has definitely gone extinct (as opposed to just being extremely rare/elusive)?

Question

How do we know that a species has definitely gone extinct (as opposed to just being extremely rare/elusive)?

In: Biology

Answer 1

I know that when you’re investigating how many different species there are in an enviroment, you can use [rarefaction](https://en.m.wikipedia.org/wiki/Rarefaction_(ecology)) in order to determine whether or not you have sampled enough of the enviroment to get an accurate look of that area’s richness. I’m not an expert, but I would imagine that a similar thing can be done with a specific species in order to tell how likely it is to be extinct