There are colorimetric methods for ammonia, e.g. by reaction with phenol or salicylate, and NH3 gas type electrodes as well, but you need a good quality pH meter to get good quality data. For total N you can use Kjeldahl.

Hello,

in order to determine ammonium, i find that the Nessler method of determining ammonium should be suitable for you. As suggested in Paul Milham's answer, it is a colorimetric assay. It has the advantage of being easy to perform (no need to heat, for urine there should be no need to correct the pH and the reaction is fast) and sensitive. Don't hesitate to ask if you have any questions. Good luck.