how do you balance a plane laterally? I mean how do you tell if one wing tip weighs more than the other? I have a seagull 540 edge that likes to roll out of loops, but I can't figure out why. I have tried to make sure that the elevator is measured correctly, but the only thing I can come up with is that the plane is not balanced from wingtip to wingtip. How can I tell if this is right? Thanks in advance for your responses.

Take the prop and spinner off and tie a string around the threads and run it back to a push pin in th leading edge of the vert. stab and pick up and see which is lower

Or... use the cradle and pendulum method to find CG.

Hang the plane form a single line, tied to a cradle. doesn't really matter how you cradle the plane as long as it hangs level.

Hang a rock or somehting from a string, tied right onto the first string which is supporting the cradle. The rock will be directly over the ACTUAL CG. (where it really is... not where its "supposed" to be.)

If the rock is off to one side... the plane is heavy to that side. If the rock is ahead of the place the plans say the CG SHOULD be... its noseheavy.

Unless one side is really out, like because of a massive wing repair where gobs of epoxy were used on one side, I wouldn't worry too much about lateral balance. It is very hard to detect small lateral balance issues in the air. More than likely, like 99% chance, the rolling out of loops on your plane is not due to lateral balance. Something isn't straight or moving properly, be it wing, stab, elevator, something.

Or you weren't level entering the loop in the first place.

Or... You just pulled to hard and are snap-rolling out.

*** will I ever quit editing this?

www.nsrca.org go to the page on how to trim-test a model. THAT will tell you for sure what the problem is.

Ya... my buddy has a problem with entering loops and Immelmans with one wing a bit low... MAJOR corkscrew to the right. Sometimes worse than others, terrible in a cross-wind, but it's not consistent enough to pin down to a problem with the airframe, and he is able to make a good one.
It is VERY easy to pull or push the aileron when pulling back elevator... I tend to roll to the left, as I fly with my thumbs on top of the sticks.