The most practical method for a right/left offset is to put something on the prop shaft, such as a stick about 12" long... 6" per side.
Then from the center of the tail post, measure to each tip. Any mismatch will be the offset.
Or fix something like the stick to the firewall and meaure from the tailpost.
for an up-down offset, level the fuselage to the datum, which is probably a line front-to-rear.
Check the plans.
Use a bubble level.
Place the stick on the firewall vertically, and using the bubble, find any deviation from vertical.
If the motor mount is installed and the motor mounted, put a piece of brass tubing on the crankshaft, again with the fuselage level, and use the bubble to see any offset.
A long enough piece of tubing can be measured at each end from the table top, and the difference will be the offset..