im curious how do you tell if you have a bad piston?

 

Compression test

 

ok thanks

 

leakdown test......is better

 

And it's harder especially for someone who's asking how to even tell somethings wrong.

 

a compression test can be done with a cheap compression test kit, cheaper ones can be had for $20. a leak down test, as it is more accurate, requires an expensive leak down tester and a compressor with a decent size tank.

 

Borescope, I'm not saying buy one but as I do often with specialty tools, maybe a rental?
Compression test or leak down test tells you a lot but about what? You find a bad cylinder
is it the rings, valves ,head gasket, hole in the piston? Seeing is believing sometimes.