NH3 is actually the "free" ammonia, and in order to test for it you need a specialized test kit like
this one from Seachem. The majority of your ammonia is actually NH4+ and it isn't toxic to your fish (well at normal levels that is). What most kits are testing for is actually a total combined ammonia level (NH3+NH4). This is fine but misleading in some ways. If you have a total ammonia of 2ppm in your tank but a low
pH (like 6.5), you won't notice your fish acting sickly because very little of that is actually toxic to them. However, the same total ammonia in a higher pH, say 7.5, would have a lot more toxic ammonia in it and you will notice problems or even deaths. This is why most of us suggest getting a master test kit that has pH, ammonia,
nitrite, and
nitrate tests in it. Its the combination of all of them that will give you the best idea of how to keep your fish healthy.