Longknife covered it well. I am no expert by any means - there are many folks on this board who are - but here are a few more thoughts of things to look for:
There have been nice antique percussion guns that have been "enhanced" by installing flintlocks, to attempt to increase their apparent age and value.
Most (not all, by any means) original flint guns used two lockbolts. However, there are a lot of examples of later flint southern mountain rifles with single lockbolts that appear to be legitimate. But a single lock bolt on a flint gun is a starting point to indicate you should at least examine the lock and mortice area very carefully.
Sometimes on old guns, the original locks were just lost, and if the locks were lost, often the sideplates too. So it is not uncommon that locks have been replaced along the way for the legitimate reason of simply trying to restore an essential element of the gun.
If a lock has been replaced, either as flint or percussion, take a careful look at the mortise. You might see that wood has been hogged out or filled to make the new lock fit. This is usually pretty easy to spot. If the mortice was opened up, often the moldings around the lock will look "off" - too narrow at some points relative to others, etc.
Take a close look at the notch in the wood behind the cock - does the slot need to be as big as it is to clear the cock that is on ther now? If there are double wear lines or the slot has been really hogged out wide and deep, it may indicate the original cock was possibly replaced and the replacement didn't exactly fall into the original slot, so they made the slot wider to hide that.
Take a look at the shape of the lockplate too - better replacements will sometimes carefully fit the metal to the existing wood mortise to leave the original wood on the gun as intact as possible. This is very hard to spot though if an antique original lock was used for the replacement.
Lockbolts moved or angled slightly - i.e. doubled/oversize holes in the wood, but not in the lockplate.
Look at the sideplate - inside and out - does it match the type and patina of the other mounts on the gun?
Good luck
Guy