Well, it would not be easy with the bike assembled, but I suppose with several pieces of high strength fishing line, some masking or electrical tape, a long straightedge, and a protractor, it could be done along these lines:
Take one piece of string approx. 8' long and tape one end to the side-view centerline of the bottom frame tube at a point that is definitely ahead of the upward curve/connection at the rear, then string it out to a point near the front wheel (perhaps tie/tape it to a spoke near the front of the front wheel), keeping the string as closely aligned with the bottom frame tube centerline as possible, taking care not to allow the string to touch anything else.
Take another piece of string approx 4' long and tape one end to the top of the steering stem of the frame, at the top, at the approximate centerline; string that piece down to the point where it most closely meets the first string, ensuring that this piece of string follows the centerline of the steering stem as exactly as possible. You may need to source something to set in this final place to tie off the string's other end to.
Now, from the side, "somehow" (good luck) measure the angle of the steering stem string, relative to the bottom frame tube string.
A good solution to the above two secondary tie-off points would be two very good helpers who can hold the far ends of the string in alignment while you measure.
