I'll be taking a trip to the CMP store soon to buy my first Garand.
Awesome, its a great experience. You will really enjoy it.
What do I need to be looking for?
What do you want the gun for? A collector's piece that will hang on your wall, something fun to shoot, an original WWII gun?
I myself wanted a good shooting Garand. So, I walked around with a muzzle guage, and guaged them till I got one less than one. I got the benefit of a SA/GAW stock, and some good metal - in places. I have no dreams of competing with my rifle, but I didn't want a gun that would shoot 12" groups either.
If you want a WWII receiver, look for that. If you want good metal, look for that. If you want a "brand new" gun, look at the Specials. As Storm said, look behind the stock. Mine is beat to snot, but I kind of like it like that - I still can't decide if I should even clean it or not. For me, I like the mystery of the history behind it - the stock could have jumped into Ste. Mere with my old unit for all I know, or been carried by my Grandfather's buddy (he was a mortarman, and carried a carbine) across the Pacific. Or maybe it was carried by a guy on a Navy ship - I don't know, but I know whatever it did, it was special.
Ask the other customers in there, and the staff. Every single person I talked with was polite and helpful. I didn't see any of the usual gun store bravado in there. Everyone was quick to help and to give pointers.
When I found one I liked, I brought it to the counter, and a really helpful gentleman who has probably forgotten more about Garands than I will ever learn helped me out. He gave me some tips and info on the certain rifle I brought to him, and checked the throat erosion.
I've heard about muzzle wear, but where can I find a muzzle gauge and are there any other gauges or tools that would help?
If you give them (at the counter) an ID, they will give you a muzzle guage. You just drop it in the muzzle and read where it stops. I am not aware of any other tools that might be that useful to you.
Welcome to the addiction - you only get to buy a first Garand once. Trust us, it is your first.