Well, I'll try to keep this simple, but It's difficult to tell which needs trim since I can't see what it's doing and since there is so much roll coupling on trainers. If I could look at the airplane I'd be able to tell you in a second. I'll give it my best shot.

I'm not sure if you know this, but...
Imagine sticking a skewer through the back of the fuselage and having it come out the nose. The ailerons control this axis of rotation (banking).
Imagine sticking a skewer through the top center of the fuselage and having it come out the center of the bottom. The rudder controls this axis of rotation (yaw).

Usually when the rudder is out of trim the wings stay fairly level, but the nose does not point in the direction the model is flying. The wings may eventually bank, but first the airplane will look like it's flying sideways at an angle.

When the ailerons are out of trim the wings just bank and the nose points in the direction the airplane is traveling.

When you take your hands off the controls does the model roll to one direction or another (banking)? If you don't have an instructor (which I would definitely recommend) my advice would be to set the rudder and ailerons to center on the ground, and if the airplane starts banking in the air, use the aileron trim. If the airplane is pointing where it's headed no rudder trim is necessary, unless you are flying in a crosswind. In that case all bets are off and the plane may yaw simply because of the wind.

My apologies in advance...I'm no artist.. [:-]