In the past I have used those little keychain flashlights dropped down the barrel of muzzleloaders. Some translucent 'scotch tape' on the lens served as a diffuser so I wasn't starring into the sun.
Primitive but it was handy and cheap.
Others have told of using their borescopes to inspect cases. If you have one then why not?
Personally, while borescopes are extremely useful in my opinion, they have condemned more barrels with long useable life ahead of them than anything else. Especially when the "craze cracking" thing was discovered by those with only a little knowledge on the subject.