Depends on the shirt. If it is a good cotton it should have been cut on the straight grain, so you could pull a thread on the cross grain of each piece (back and 2 fronts) that SHOULD be even!? I'd check it be trying to pull a thread very near the original hem to see if it works.

Anything more fool proof? - Make a new shirt from scratch :-P

Good luck,
AJ

You should be able to cut the back straight across by measuring from the sleeve to the length you want the shirt to be cut on both sides (This would match the front length at sideseam). From there draw a line from side to side and cut! I would also fold the jacket along the center back with the fronts facing out. Get the two sides as even as possible to see if the places marked are even. Sometimes the armholes might not be exactly the same depth and this would make a difference in the first measurement marked. If this checks out, you are ready to go!

Okay ladies, I'm going to take a stab at this during the week!! I'll let you know how it turns out!
Thanks so much for your time in answering!