Well, there may be one way to determine if they are real or not...
Remove them, and look at the back side. It may give some indication of mold marks if they're plastic.
Or, try giving them a scratch test. Plastic will often scratch and leave a rooster tail curl. Horn, being hard and bony, won't.
Another possibility, heat a paper clip red hot and touch it to an inconspicuous place on the back of the grips. If you get melting and plastic smelling smoke, you know what they are.
Stag horn, on the other hand, probably won't smoke at all, or very little, and will just show a dark mark.