To get back to the original question... I'm not an expert, but I think this is the answer:

In practice, with a plane built reasonably straight, it doesn't matter. Just trim the ailerons and fly. Personally, I trim the rudder with a straightedge on the ground, and never trim the rudder in the air.

In theory though, it is still an interesting question. I think the answer is that if you use the rudder to trim to straight level flight when you should have used the ailerons, the plane will be crabbing (i.e. the tail is not exactly on the line of flight), and you could probably notice it from the ground. In other words, the ailerons are off, but you compensated with the rudder, which would work in terms of leveling the wings, but it would throw the tail off whack.

Also, Fastsky, could you elaborate on your statement that, "Planes with no dihedral can not be turned with just the rudder." I fly planes with no dihedral, and I'm pretty sure if I use the rudder the plane is going to turn (other things might happen too, but it seems the plane will turn).