I'm not sure if my submersible water pump is a 110 or 220 volt. If I get the voltage wrong in this, can I destroy my pump?
The pump has a durable data plate on the side. Easy to read with the pump out of the well. If the pump is already in the well, how was it powered previously?
Pump was already in the well, so I don't have access
So how it was powered before? Why are you asking about 110 or 220? Or is this a new well, you dropped the pump down the well and didn't look at the pump's label? No instructions?
You have to know what it is... Look at the well head. Many installations place that information at the top for this very reason. As jreagan points out, the other hint is how the thing was originally wired.
Note that it's not even necessarily straight 120 or 240.
I have a variable drive pump in my well. While the controller is fed with single phase 240, the actual motor in the well is 120V three phase.