They have gages to check the muzzle wear. Though they may be pricy for "one' rifle.
A trick us old cheapies use is to take a M-2 loaded round. (the one with the 149 grn bullet).
Stick the bullet end, into the muzzle. If it goes in all the way to the rim the barrel is shot. If it goes in to where its 1/16 from the mouth of the case, you have 1/4 barrel life left. 1/8th, in from the mouth of the case, you have 1/2 of barrel life left, 1/16, you have 3/4 barrel life left. !/4 should be a new barrel.
This is a ruff estiment of the muzzle end and gives you a guide if you dont have gages. It wont tell you about throat erosion.
The thing about these old military gas guns. They were cleaned from the muzzle end with jointed cleaning rods. More were cleaned to death then shot to death.
Of course no method of testing a Garand, or any other rifle beats a good old fashion "shootin' test".