One method of finding the concenration of ammonia if it is in solution is to tritate it with a standardised acid solution, e.g. oxalic acid. This is because a solution of ammonia is basic because it produces ammonium hydroxide, and therefore hydroxide ions.
NH3 + H2O >>> NH4OH >>>>> NH4+ + OH-
Then it is possible to calculate the concentration of the ammonia solution, using the tritation calculations beacuse you are just performing a tritation.
If the ammonia is free, then I'm not sure, maybe electronic sensors.
Procedures that could replace tritation may be calculating the density, because the density of the solution would change as more of the solute is added. But this method would likely be very inaccurate. Other than that, I'm not to sure.
Hope that this helps,
Jamesy