Pull the spark plugs. Use a borescope through the plug holes to inspect the combustion chambers. The one I use has a probe 5MM in diameter, and fits through a .22 caliber rifle barrel. It's even got a 45 degree mirror attachment for looking sideways. I think I gave less than $50.00 on Ebay for it, and it paid for itself the first time I used it on a mechanical inspection because I didn't have to waste half a day sitting around while someone pulled a cylinder head to confirm a damaged piston.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!