Carbines can be tough, due to the number of manufacturers, sub-contractors, and the commercial builders. Parts were also upgraded throughout its service time.
Most major USGI parts will have manuf. code on them, especially of course the prime contractor which should be listed on the receiver as mentioned above near the serial number. (IBM, Underwood, Winchester, Rockola, Saginaw, Inland, Quality, National post Meter, Standard, Pederson/Saginaw Grand Rapids, etc. (10 ttl)).
Some main contractors used barrels from another prime contractor, so a different barrel builder mark (marked near the front sight) isn't neccesarliy bad. (like NPM using Underwood bbls)
Major parts like the stocks, hand guard, slide, the rear sight, the trigger housing, mags, etc. etc. should have builder markings, like "W" for Winchester or "U N", showing it as is a sub-contract built part (there were dozens) for a prime builder, i.e. Union Hardware or Union S&S (U) for National Post Meter (N); or "OI-B" for an IBM (B) subcontractor, SG or S'G' for Saginaw, I for Inland, U for Underwood, RMC for Rockola, etc.
Original WW2 builds are of course worth the most - they should get top dollar, & if in nice condition are hard to overpay for. Some prime contrators are rarer then others...S'G'/Pederson, or are more desirable 'just because' (Winchester). The stock would be cartouched, the manufacturer codes should be correct through-out, part versions should match the date produced, etc.
US arsenal re-builds (all parts updated to the latest by the gov) should get a also decent price - they have collector interest and are fun 'cause they have the bayonet lug and better (rotating) safeties and mag catches - look for an arsenal rebuild cartouche/stamp on the stock and all USGI parts. A DCM gun should have papers to confirm it's history, otherwise it MAY be considered just a "parts gun". Those that have been re-imported into the country will usually have an import mark on the barrel somewhere, and aren't as desirable. Nor are 'parts guns' where someone has mixed and matched things, though still USGI - are good shooters, but should not get top $$$.
Only Inland & Winchester ever built an original carbine with a bayo lug late in the war, so a band with a lug would typically indicate a re-build. Most originals also had simple flip "L" rear sites, so check that too. If the bayo lug has been 'restored' with a older no-lug type I/II band, there is often a bright spot in the finish about a palm's width away from the band up the barrel. If the rear site has been re-replaced with an older L-type one ($$$ these days), there are usually stake marks, or even double serial numbers from the newer adjustable sight install. Early (most original) guns should also have a flat bolt (blued), a 2-rivet hand guard, push-button safety, high wood stock, simpler mag catch.
Many original barrels are still attached to their receivers and in decent shape as the carbine ammo was not corrosive.
You certainly want to start by identifying the builder on the receiver. USGI is a must. You then want to see if you can tell - is it orignal, an import, a rebuild? Is it a DCM with papers? How is the bore? Can you take it down to check part codes? What do YOU want?...a collector, a shooter, a project gun?
Good luck!
