Manufacturers names will be stamped behind/under the rear sight. Sometimes these can be hard to see.
Barrels are often stamped below the front sight with a makers name and often a date.
Typical USGI makers were:
Inland Division General Motors.
Winchester.
Saginaw Division General Motors.
Quality Hardware.
National Postal Meter.
IBM.
Standard Products.
Rockola.
Irwin Pedersen.
Companies that ARE NOT GI include:
Universal.
Plainfield.
Iver Johnson.
National Ordnance
Most carbines will have been rebuilt so barrels and receivers may not have the same makers name, and barrels that were replaced often didn't have dates on them.
Look for any importers stamps on the barrel, usually on the side or bottom.
Names like Blue Sky, CAI, and others are importers of carbines that were given/sold to foreign countries like Korea, and have been re-imported.
There's a WORLD of collector's info on "correct" sights, bolts, bayonet locks, stocks, etc. but most any genuine USGI carbine in any kind of shape will bring $400 minimum.
One in really nice condition can go for $600 and up.
One in REALLY nice condition can go for over $1000.