I have a US41 (magneto & spring starter) engine in a plane I am restoring. I need a little help. It has a wire going from ground and magneto to a single toggle switch.

I need a tip on how to test it with a meter or something to tell when it is "hot" and when it is "off". thanks.

Take the spark plug out, hook it to the lead and ground it to the engine. Flip the engine with the spring start and watch for spark. If it does not have a spring start use an electric starter to make sure you have enough rpms for spark.

Hmmm, take a multi meter or if all else fails, a 9 volt battery w a test light will tell you when the switch is open and when its going to ground to shut off the engine, you don't need to crank over the engine, you just need to know which way the switch is open ( the run position) or closed ( the off position)