The first thing is to understand the difference between free and total chlorine. When chlorine is added to a water system for disinfection, it will bond or react with the bacteria, germs, and nitrogenous compounds present. Some of these bonded species are chloramines.
Total chlorine is the sum of the free chlorine and chloramines present.
Perform a total chlorine determination and a free chlorine determination. Then subtract the free chlorine result from the total chlorine result. That yields the chloramine concentration. A test specifically for monochloramines is also available. When I add the DPD reagent, the solution turns colorless instead of pink.