Braxtons are essentially tightenings. So you will be able to see it happen as well as feel it. My placenta is on my right side and hnni have one my stomach wraps round it and my belly looks lop sided. Basically you will feel your stomach tighten, as you get further a long your breathing will be a bit laboured and you may suddenley feel the urge to pee- this is cos of the pressure on your bladder. After about a min you will feel and see your stomach relax and become soft again (your stomach can become very hard durin a braxton).
Bare in mind that not all women will have braxtons, especially for a first time baby...and you in fact can go without noticing them-so they are very tiny contactions. Mine are very intense, and nowadays I get more contractions than ido braxtons.
braxtons are always irregular in time and length. So the minutes they get themselves into a pattern then you are most likley having contractions...especially if they last a minute or more, and if the are followed with pain. (braxtons tend to be painless)