How do you know if the spark plug is bad on a weedeater lawnmower?

Remove the spark plug,be very careful to keep socket strait so you don't break spark plug.inspect plug,check for cracks excessive carbon buildup,clean with wire brush and solvent,gap should be about.035 of an inch or fold a match book cover over two times thickness of three reinsert spark plug tip in rubber boot spark plug wire,hold metal part of plug to metal part of engine,engage safety handle and pull start rope,you should see a little blue spark across the gap,be careful to only hold rubber boot or you might get a shock,non lethal,just doesn't feel good.It kind of tricky you need two people, or you have to tie the safety handle down so you can pull the start rope and hold the plug to metal and pull the cord and watch for the spark.if your in doubt just buy a new plug for two or three bucks from a hardware, or auto parts store,have them gap the plug,they should be gaped from the factory but it's a good idea to double check,and install the new plug don't over tighten,or cross thread,screw it in by hand,then final tightening with socket and ratchet. REMEMBER (CLOCKWISE ROTATION TIGHTENS)(COUNTER CLOCKWISE LOOSENS) have fun, trouble shooting small engines is so much fun!!