The Robart incidence meter can be used to check for and measure +/- engine thrust. But what about side thrust? Assuming it's not so off as to be easily visible to the eye, what methods are there for checking side thrust?

Firebee (Mike P.) offered one technique, namely measuring the distances (on both sides) from the prop tip to the rudder post with a piece of (on non-stretchable) string. Are there any other methods?

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I just tried Mike's method using a length of wire cable with a small hook that snags the edge of the "tail post" (which technically my SE5a model doesn't have since it uses a hidden lever to actuate the elevators). I then pulled the cable taut up to the prop tip marked the spot on a bit of tape, rotated the prop 180 degrees and checked that it was at the same spot (as Mike says in case there is an issue with prop symmetry). Then I connected the cable on the other side and measured again.

And the distance is, indeed, about 2mm shorter on the port side, meaning there is some degree of bad left thrust. I'm not sure how much this amounts to. I suppose it could be worked out with a few more numbers and some geometry. But it's enough to know that it's there and needs to be corrected.

Unfortunately this won't be quite as easy as placing washers under one side of a mount since the engine is mounted on beams built into the model. But it's doable.