Couple of things.
For in-out thrust
If the fuselage CL is is straight & perpendicular to the wing.  Push in a straight pin at top of the vertical with a long string.  Put a prop on the engine, horizontal.  Use the string to measure to one of the prop tips, and put a mark on the string.  Rotate the prop 180 to eliminate any variations in dia, etc, then measure to the tip on the opposite side.  If the distances are equal the engine thrust line is parallel with the fuse CL in the yaw axis.

Measure from the same prop tip to the LE (straight LE), or TE (straight TE) of the wing on both sides of the fuselage at equal spanwise point.  Equal measurements, 0 engine, as above.  This could work with a tapered LE IF the taper is identical on both sides.

The string can also be used to see if fuselage CL is perpendicular to wing TE (LE). Measure to a point equidistant from fuselage CL on the LE or TE.

For up-down thrust.
An incidence gauge is the best.  Fix the fuselage to a bench, set the wing at O degrees.  Then with a straight edge clamped to the motor mount (engine out) use the gauge to see up or down.

Thanks Don, as soon as I read your reply I remembered doing that on r/c pattern planes. I have long sticks made
special for checking engine offset. I checked my Ringmaster and it was real close to perfect, the Twisterized
plane needed a .010 shim under the engine lug to point it straight and the P-51 had a little out thrust so I left it
alone. I have to get to the flying field to test all the changes I've made lately.
Perry Rose