Interesting - I was told to use protein now and then, but it wasn't so specific. I wish it was!
Anyway, this is what I know about proteins (and it might not be all-encompassing):
if it says "hydrolyzed" it's probably a protein
if it says "amino acid" it's probably a protein
if it says "keratin" it's probably a protein.
HTH! Hopefully someone else will be able to tell you more.