You see a 1911 barrel at a gunshow that is claimed to be "GI".  Is there any way to know for sure if it is the real deal?

Well, in general, any barrel that has been throated for better feeding is not "GI" although many users have had this done to their GI barrels. The late ones are also chrome lined, but I don't recall them being specifically marked as such unless there is a single letter code stamp. I have one at home that I know is the real thing because I pulled it from a 1911A1 a friend's father got via the NRA many decades ago (for $17 I believe Ha!), and will check the markings today if possible...tomorrow for sure...and report back. I am not familar with any means of being able to verify that a specific barrel was made to GI apecs, but there may be some list of makers and marks somewhere out there.

One of the most informative websites on the Model 1911 is http://usgi1911.tripod.com/.
There is a wealth of information there.