Take off the rocker cover, pull the plugs and turn the crank by hand with a socket and extension bar. If you see any slack on the rockers as you turn the engine you have found your dud valve. You might have to grab the rockers and jiggle them by hand to feel for play if you can't see it. It is easy enough to do and is quick. Other options are borrow a compression tester from a friend and see what prevails and/or just look at each individual spark plug and see which one stands out from the rest.