The easiest way without taking anything apart is to pull back the cocking handle and look to see the marking on the rear of the chamber. .30 is .30-06, 7 is 7mm and 7.9 is 8mm. You can also take the magazine out and see what is stamped on the side of the follower and of the front locking tab of the magazine box. There are other locations, but these are the easiest and most marked locations for all of the contracts.
Cheers,
Ian