That would be the same as saying you'd have to prove every antler used in a pen or knife was taken legally.
In Illinois it's illegal to possess antler on skull plates if you don't have a deer tag or road kill tag to prove it was legal. Also, it's illegal to sell road kill antler. Only sheds or hunted deer kill antler, and
if you cut the rack off a roadkill you only have 24 hours to report it and file for a tag. We have a TON of deer in IL and roadkill are pretty plentiful during the fall and winter season. I'm guessing it's pretty hard to stay up on these regulations unless you stumble across it while dealing with something else. I keep reciepts and tags for everything.