I just bought my first M1 carbine through the CMP. It hasn't been delivered yet. I'd recommend you get one through them. A rack grade Inland can be had for $419. Rack grade Underwoods are $479. I got a rack grade Winchester.
I'd say the most important thing to check is the muzzle wear. You can use a gage but a .30-06 bullet works just as well.
Their site has all the info you need. Be sure to check out the article on M1 magazines.
http://www.odcmp.com/rifles/carbine.htm