Good question!
Here's what little I know. Most gov't rebarreled M1917s will probably have either a 4-groove Hi-Standard barrel or a 2-groove Johnson Automatic barrel. The two groove barrel should be easy to spot and the 4-groove barrel, if it is a Hi-Standard barrel, will have a
right-hand twist. Again, that should be easy to spot.
Of course, there could be many exceptions to the above that I'm not aware of. I also believe the barrels will be marked, like JA, for the Johnson Automatics. I'm not sure how the Hi-Standards were marked.
To check the receiver ring, you need something like lighter fluid or alcohol. Paint the ring with either fluid and watch it evaporate. If cracked, the fluid in the crack(s) should evaporate slower than the fluid where there are no cracks. Of course, this needs to be done with the cooperation of the seller.
Hope this helps.
--
Mike