Absent some easily verified accomplishment, when someone lacks humility, I generally assume they're stupid to protect myself. They might be smart but it's going to take a lot of effort to prove it if they desperately and consistently elevate themselves in trivial matters.

Otherwise, they consistently produce better and original thoughts/insight given novel information and can layout their reasoning in an easy to understand way (or it's obviously insightful in retrospect) so I can verify their conclusions.

It's this verification part that's key for me. Lots of people have trained themselves to deceive people into thinking they're smart.