ORIGINAL: MeanV2
Form a T with your arms and body back up against a wall. Measure from finger tip to finger tip and divide by 2.5. While the results may not be wrote in stone, it will give you a good place to start and it could be right on the money.
Dan
Thats a great way to get close. And going in blind thats what I recommend.
The bow method that ButchA refers to works... if you have decent form and keep your head up straight.
What Dan writes illudes to the real truth... and that is that bows are just like shoes.... a size 11.5 Nike might be the same as a size 11 New Balance (infact it is because thats what I wear... Nikes always run small). There is really no set in stone standard as to what constitutes 11 vs 12 etc... just roundabout guidelines.
And so it is with bows.... Bear... for example... notoriously short, especially when you get out to the longer draws 29"+... in my opinion its a poor design because they give 30" and 31" draw shooters nothing but problems. High Country.... runs a little long... I have a Mathews LX as my backup bow set at 29" and its a hair short... my High Country Iron Mace is set at 29", but measured AMO/ATA it is really 29.5" (which is my draw length as its measured).
Don't worry about hitting it right on the head... all I ever use it for is a starting off point...I adjust it til its just right for the individual.... any tech worth their Allen wrench set will do the same.