LED monitor for solenoid
How much current does the solenoid draw?  (what's it's resistance?)  If it's low enough then maybe the LED in series will work.  If not you'll have to monitor the current and turn the LED on when it's flowing (as tcrosley did with the 1 ohm resistor.)  







simulate this circuit &ndash; Schematic created using CircuitLab

This may be a bit simpler.  You have to choose R1 such that it drops ~1.0 Volts for the solenoid current. R3 then limits the current to the npn and R2 sets the LED current.  This assumes that the solenoid can float.  If not then a pnp on the high side.