What's the best way to test a fuel injector on a Merc. 502 MPI ecm 3? I don't know if the injector itself is bad or if it isn't getting a voltage signal. Can I hook a voltage meter or an LED to the wires of the injector, crank the engine and look for a voltage signal? (There are no error codes.)

the injectors should ohm about 10 to 13 ohms, you can use a test light across the injector wires, just dont let it run long like that, look for a good pulse of light from the test light

Trust me...injectors can destroy a motor. I'm re-building a motor now (with some buddies) that detonated and "came apart" after 2 hours due to bad injectors.
I found out the injectors were my problem by sending them to Marren Fuel Injection (injector.com). He was able to test them and tell me which injectors were the problem.