Bore scopes are used to check internally. There is only so much - and sometimes, quite a bit, that can be learned with a lubricated, tight fitting patch.
On a muzzleloading gun, the breech should be pulled to get a good look, full length. If that cannot be done, then a bore scope - if not available, then dropping a small low powered flashligth down the bore and looking can work - as well as the tight patch on a jag mounted rod will tell lots, is one knows what to 'feel' for. Roughness, expansion (generally see that on the outside as well) rings causing short loose spots - crunchy badly corroded areas - ya know, the usual stuff.
If a muzzleloader looks good on the outside, chances are it's been well taken care of. There is very much less chance it had an outside overhaul to trick the buyer, as is done so often with ctg. guns.

Being older and of less 'desirability'(marketability) on the market, muzzleloaders are usually sold 'as is'. What a buy!