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.