How do i know the difference between braxton hicks contractions and preterm labor?

The location matters: If the contraction can be 'located' to a certain part of the uterus, and not the whole uterus, then it is a braxton-hicks contraction. If the entire uterus contracts, it is a true contraction.

Pain & frequency: True labor is generally painful, or at least associated with pelvic pressure and/or backache. True contractions come every 5-10 minutes, whereas false contractions of braxton-hicks come every 30 minutes. If in doubt, consult your obstetrician asap.