Will a leakdown or compression tester tell me if i have a non seating valve?

I get the general idea how one works, how do you tell if its a valve thats not sealing?

 

Yes...

 

which one who be better to use

 

The leak down test will tell you if your valves are not sealing. But do both while you are at it. Why are you concerned about bad valve sealing?

 

i had a nasty header leak and i had a rr jump off the #3 exhaust cylinder,
every since then the car shakes/ misses at idle , and has some valvetrain noise

I figure ill check the vacuum , and then do a leakdown test

 

did you replace the rocker? are they NSA or SA? not sure about the noise maybe just needs more preload

as for the missing, could've foulded the plug on that cylinder

 

after the rocker jumped off
i switched from crane 3/8 sa to promag 7/16 nsa, plugs were all pulled i ran it with 1/8 turn past and also tryed 1/4 no difference

i went thru every sensor on the car, motor mounts, plugs,wires etc, so i think a leakdown test will help tell me whats going on