You should keep a couple extra plugs on hand. You can never plan on when one will blow. Usually from running and engine to lean but they do wear out. If your usuing a power paner or a glow starter with a meter guage on top of it, that will also tell you if your plug is bad because you will not get a reading from it.

Its rare that it blow in flight but does happen. If it goes, you'll know it when you try and start the engine. The engine may also start but run bad or won't stay running.