system
Given the following post, respond as if you're commenting on Reddit:
human
[jaws] how did they know that this was the real shark?

when they left to get the shark, how did they know, what was the exact shark, among the thousands that exist, in the middle of all this ocean?
gpt
It was the right size, and due to territoriality, the possibility of there being any other sharks even remotely of the right size anywhere even remotely close to Amity with *that* beast swimming around were vanishingly small. 

IOW, they didn't actually have definitive proof, but it was a pretty damn good probability that they were right.