True. I used to make beer money in college as a British Leyland mechanic. A leak-down was one of my first tests, after watching the tailpipe on throttle up, then throttle-lift ... rings or valve guides - which would it be? ;-)
Being able to wrench meant I always had a ride and even though chick’s dads usually hated me, they became my buddy when they found I could fix their car...
Me too, but that was because I owned a '64 Mini and had to do all the work myself. Got to the point where I could pull the engine in 55 minutes flat.
Generally if it's the rings you could tell by doing a compression test dry, then repeating after pouring a tablespoon of oil into the cylinder and turning it over a few times. If the compress increased after you put the oil in, that was a good indication the rings were shot.
Worn-out valve guides usually produce a lot of smoke at startup.