If the butt cap is Ivory - you'll see small rings/veins in it.
If it's Delrin, a plastic material which came out in the 1950's and is used on Swiss Army Knives...advertised as a substitute for Ivory, it will melt at around 200 degrees and has no veins. (don't try boiling your Swiss Arm Knives to clean them....they will be handleless in no time). (and if the cap is Delrin, don't hold a cigarette lighter up to it).
There is a substance called "Alternative Ivory" which came out in the 1990's that looks so much like real ivory that knife makers such as Randall, specifically identify it so as not to fool their buyers.
Randall Knife with "Alternative Ivory" handle:
