theres a couple of ways to check valves; im guessing you can get some kind of general reading with a compression test; leak down test, not cheap; disassembling head; having a shop check valves; i take a med/small sheet of glass, not thin, about 10"x10" should do, take valve and lay down on glass(as glass is perfectly flat), roll valve, stem and margin sit on glass as you roll; a bad valve will wobble. of course any valve that has made an impression in piston top is no good; try glass-rolling trick on these and see how it works.