Other mistake: In the beginning of the film where the truck with the machine gunner and the driver is shot, the truck goes out of the road and falls over, but there is no machine gunner falling of the cargo area nor is there any driver falling out from the truck.
Other mistake: In a whole lot of scenes, Bob closes both his eyes when he fires his sniper rifle. This is a common and natural reaction when an inexperienced shooter fires a gun. An experienced hunter or marksman would never do that, and especially not a military sniper.
Other mistake: When Swagger is dialing in his sight to shoot the Denty Moore stew he is dialing in the windage but they show the elevation moving.
Other mistake: If you look at the license plate on the government car that mark Wahlburg is driving, they are not government plates but regular Pennsylvania license plates. The next scene the driver tells the FBI guy that they are government plates which they are not.
Other mistake: When Michael Pena is looking at the photo of Swagger after winning the Wimbledon Cup, it identified Swagger as "Sgt Major Bob Lee Swagger, USMC, from Gillette, WY." Two things are wrong: (1) Swagger is wearing civvies, if he's in the Corps, he should be wearing his uniform; and, (2) Swagger is supposed to be a Gunnery Sgt, not a Sgt Major. (00:44:20)
Other mistake: When Swagger shoots a driver through the windshield in the opening scenes, there is no hole in the windshield.
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.