there are two ways ,one: is by pH meausuring in situ ,you need to see a pH disminution or by taking an aliquot and make a titritation, right now i don't remember the indicator reagent

best regards

If you dilute the solution with water (or extract the soln. with water), the ionic bromide from HBr can be detected easily with silver nitrate as AgBr (light yellow) ppt. which can be dissolved in cc. NH3.  Similarly, lead(II) and Hg(I) salts gave yellow bromide ppts. However, the starting organic materials should be tested with the metals alts whether they gave ppt. with similar color or not.  

This is just a guess:  add an oxidizing agent -- a mild one -- to convert the bromide anion to orange-red Br2.