I'm sure you could draw 2 lines 6" apart on the back and it would be a little longer at brace.
You wouldn't be able to easily measure the difference over a few inches, but it would be measurable from nock to nock. If you take something easy to measure along the back like a Hill longbow design with several inches of reflex unbraced there will be around a 1/4" (or more, depending on the details of the design) increase in the nock to nock length from unstrung to braced.
Mark