How do I diagnose a stuck valve?

 

Take the valve cover off, and rotate the engine by hand. Observe valve movement.

 

I've seen valves stuck in broken engines when remove the head. If the engine stay together it not rotate!!

But what most people call stuck valve in running engines is a bent valve. Basically a compression or leak down test should tell you.

 

Compression test done
Results...
#1 - 120
#2 - 130
#3 - 130
#4 - 150

#1 is where the noise is coming from