The channels are the following in most (if not all) 4 channel airplanes:

Channel 1: Ailerons
Channel 2: Elevator
Channel 3: Throttle
Channel 4: Rudder

Then there are things like:
Channel 5: Retracts (landing gear)
Channel 6: Flaps
Channel 7: Spoilers or airbrakes

The first four you should memorize rather quickly if you have to take apart your trainer as much as I did (don't ask). Knowing where to plug in which servo makes reinstallation much faster (obviously).