Or there's a simpler less informative test to determine whether it's rings or valves.
Do dry compression test, then one cylinder at a time, squirt a little oil in the cylinder and repeat the test. If the compression comes up it's rings, if not it's valves.
Don't use too much oil or the test is useless, just use a squirt or two, enough to help seal around the rings. It can give you a place to start.