The proof marks and the quality of workmanship should be a good indication. For example I once owned a belgian percussion shotgun labeled simply "Parker". The name was a take off on William Parker who was a quality british maker. On the other hand the gun carried belgian proofs and the workmanship of the locks on the inside was much rougher and less finished that would be found on any but the least expensive british gun.
I have owned a number of muzzle loading shotguns and viewed a bunch more. While it is not particularly uncommon to find belgian guns with apparent british names on them, the name is usually not the complete name such as the Parker above rather than W Parker or W Parker and Sons. Also both the UK and Belgium had proof laws which required the proof marking of their guns and such proofs were usually fairly deeply struck. The quality of workmanship, particularly of the locks on the inside, would be the biggest tipoff to a forgery.
To me, it seems unlikely that a Wilkes would be falsified from another british gun because I don't think its value would be high enough as opposed to creating a phoney Purdey or J Manton.
While I think of it, some names were simply other makers of the same name rather than forgeries. W Richards for example was William Richards as opposed to a forged Westley Richards. On another list, some people will immediately claim a low level W Richards is a cheap belgian copy but that is not the case, it is simply another maker with a similar name
cheers mooncoon