There's a couple tools that'll work, some are costly. I have a tool made by Snap-On that's nothing more than a very small bulb at the end of a flexible wire, powered by batteries (like a remote mini-flashlight) and it was only about $20 15 yrs ago. The bulb fits inside spark plug holes and I can see inside with my bare eyes using this. I don't know if you can buy anything like that today elsewhere.
For ALOT more money, there's a device known as a boroscope which is made for seeing details inside a cylinder w/o taking the head off. Most auto shops have one.
As far as it being a head gasket, highly unlikely if never ovedheated but I've seen and heard of cracked blocks (also rare) and if it's cracked, it'll be cracked between two cylinders and only visable with the head removed. A cracked blocked is best cured by replacing the engine with a reman so hopefully that's not your problem.