API (aquarium pharmacuticals) or SeaChem. There's a few other good ones out there, but those 2 I have personal experience with. Or you could go all out and look into the wonderful world of electronic testers. But those do cost an arm an a leg.
I use API and they tell you to shake vigorously. well when i do this the freakin top leaks. i have no idea how accuarate it is then. probably still is because its only a drop or two but seriously, how is that effective?!