Usually, the dead giveaway will be the shape of the magazine. .303 Ishy's have the familiar, rounded SMLE type and 7.62x51 a more "squared-off" look. Also, it will be marked with the model designation on the "collar" where the butt stock attaches. As was said, if it's marked "No.1 Mk. III*" it will be .303; if "No.2" or "No.2A1", it will be 7.62 NATO. Caliber should also be marked on the collar. It is on my own No.2A1 and .410 "Prison Musket".