Yes, you can test the solenoids without injecting nitrous into the motor. I forgot exactly how to do it though, it's been a few years...but I have done it in the past. If you have your manual & instructions that came with the nitrous kit, it should explain how to do the solenoid test in there. You just unhook one of the wires on one of the solenoids...sorry, not sure which one of the solenoids, nor am I sure if it's the + or - wire that you disconnect. Of course, you could always just disconnect the nitrous line that goes into your throttle body (if that's the application you have?), leave the solenoids connected and test it that way. Just make sure you are in a well ventilated area.
Not sure if I was of any help...