The way I tell if a gas gun has an out-of-square bolt face is I use the trimmer on my Wilson case trimmer to hold the case backward and concentric and touch the case head. It's 1/2 wide and if it cleans one side before the other, the head has been fire formed out of square by the bolt face. I got lucky with both my M1A and my DCM Garand, as the Wilson marks their heads both evenly. A couple of other Garands I have for as-issued matches didn't do so well, and the as-issued rules don't let you do very much to the gun, so that was catch as catch can.
Whether or not you notice concentricity is partly about bullet length and shape. Short ogive, long-bearing surface bullets don't have room to tilt much in the bore. By way of comparison, Harold Vaughn got 0.004" of bullet tilt in a 6mm to open groups about 1/3 as much as A.A. Abbatiello measured it to do with the .30 cal M1 Type bullet.
Whether or not you notice concentricity is partly about bullet length and shape. Short ogive, long-bearing surface bullets don't have room to tilt much in the bore. By way of comparison, Harold Vaughn got 0.004" of bullet tilt in a 6mm to open groups about 1/3 as much as A.A. Abbatiello measured it to do with the .30 cal M1 Type bullet.