Quoted from e3thomas:What is the best way to check the circuit?
That depends on the circuit. For relays and solenoids that don't pull in when they should, I recommend Alligator Clip Jumper wires.
http://www.planetimming.com/Pinball/troubleshooting/EM%20Troubleshooting.pdf
http://www.pinrepair.com/em/index3.htm#features