Do you have a Ouija board?
Seriously, every manufacturer seems to be a little different. The style, cut, and fabric content also play a significant role. The starting point would be to measure your waist, hips, and inseam, then go to the manufacturers website and see which sizes are closest to your measurements. If the jeans are 100% cotton, consider going a size larger because they will have zero stretch. I prefer jeans with at least 1% lycra (2% is much better), because they provide a snug, yet comfortable fit. Good luck.
P.S. - Don't expect to be able to use the front pockets, they are usually quite shallow.
Love always -- Jessica Rose