This is a discussion on stag grips within the Handgun Accessories forums, part of the Pistol & Revolver Forum category; how do you determine if you have stag grips ?...
Look at the backs. One solid color with a very smooth surface tells you it's plastic. You won't see that on antler. Plus the fact that they just look "molded".
Once saw a guy on TV take a small scrape from the back side and set it on fire. If they are stag or bone (or any organic) the smell is the same as burning hair.