Iv'e noticed that some of my planes look like they are "crabbing" through the air- they fly straight after they are trimmed, but seem to be skewed slightly. I was wondering if this is more a function of thrust angle, or rudder trim. The reason I ask is that on some engines, especially Evolution 2 stroke glow, the sides of the engine case are not "parallel" underneath the mounting tabs, so there is a possibility of mounting the engine slightly crooked.

Is there an easy way to tell if the engine thrust line is correct? The planes seem to fly just fine other than the issue i'm describing.