Mr. Rate: Would've been a bad job to take, though.
Nick Memphis: How come?
Mr. Rate: Whoever took that shot's probably dead now. That's how conspiracy works. Them boys on the grassy knoll, they were dead within three hours. Buried in the damn desert. Unmarked graves out past Terlingua.
Nick Memphis: And you know this for a fact?
Mr. Rate: Still got the shovel.
Bob Lee Swagger: I was kinda hopin' to keep one of 'em alive, but they were determined to kill you.
Bob Lee Swagger: Not bad for a poodle shooter with a makeshift silencer, huh?
Answer: He's looking for callouses on the hands. Professional snipers end up getting hard skin inbetween the webbed part of the skin between the thumb and forefinger because of constant rubbing on the trigger guard. By checking the hands, he could tell whether or not he was talking to a proper sniper, and in particular Swagger.
GalahadFairlight
The webbing between the thumb and forefinger would rub on the stock or grip not the trigger guard.