To find the dud one, check them on a lawnmower, take the mower plug out, use a jumper lead from the spark plug lead connection to the spark plug thread [so you have contact] then pull the starter cord. The one that doesn't spark is the dud.
Another quicker way is to use a multimeter and check for discontinuity between the spark plug body and the firing tip - this sometimes doesn't work, but it is a quick check and if you find one that has circuit then it is the dud one.