system
Given the following post, respond as if you're commenting on Reddit:
human
What are the criteria for having a species declared extinct? How are scientists so sure that there is not one single animal of a species left?
gpt
Basically you have to have a bunch of different teams gathering data over quite a few years on last known sightings and animals that have been tagged, etc. 

After that you have a committee come together, review the data, have the data proven, and after a while they'll agree the animal is extinct. With some it's a lot easier than others (kinda hard to miss elephants or rhinos if they still live in areas)