I have never had a positive result for phosphates with the tests I've done, but I know if I start adsorbing it, my algae starts to brown and recede, so it must be there. Apparently most of the test kits only test for orthophosphates (saying that just like I know what it means), so they aren't very good at determining actual levels.
The
Hanna Colorimeter is said to be an excellent way to test, but it is an expensive thing for a low-level hobbiest like me to afford. If we had a proper "reef club" locally, maybe it would be the kind of thing to have as a lendable resource.
I've stopped testing for phosphates. I'm concentrating on getting rid of it instead.
Cheers,
Matt
<edit>
Re-reading Randy Holmes-Farley's
"Phosphate and the Reef Aquarium", recommended to everyone interested in the phosphate discussions that have recently sprung up all over the board. Stick with it to the end!