Photo credit: Maridav iStock
I had never seen this before but this probably works.
Grab a pair of pants and see if the waist band just touches when you wrap it around your neck. If it does, the jeans probably fit.
This is based on an old theory that your waist is twice as big as your neck. So one side of the waist band should go around your neck once.
Also your neck is twice the size of your wrist. So you may be able to use that to your advantage.
Or you can use the tried and true method of taking them home, seeing if they fit, and then returning them if they don't!