If you want to get technical, you can test the water output for Nitrates and Phosphates if you use a reactor of some sort. However, certain brands actually contain phosphates so make sure you're using a reputable brand. I know that Two Little Fishies have been tested and have 0 PO3. Do a search and you can find out what ones contain it and what don't.
And I read somewhere its 1lb per 100G if you wanted to know how much.
...and be careful because introducing a potent carbon in a full dosage such as snowboarder stated will clear the water so quickly that photoburn WILL occur. Happened to my corals anyway. You can reduce the lighting period, raise the lights, or ramp up the dosage gradually in order to avoid problems.