There is a test for carbon, but from what Richard said, I wouldn't recommend it. Its basically a dissolved organic (DO) test. I just change mine out once a week or every other week. I used a powered carbon filter but what you are doing is probably ok. I assume you put the carbon in one of the chemical trays in the back of your tank. If so, that's good. As long as you have flow going through it.

Running carbon will help you manage the cyano (the red slimy stuff) you have in the tank. Cyano likes water with high DO and low flow. I know you already added an extra pump. As I mentioned, my first tank was very similar to your's, and looking back on it now (with a lot more experience) I realize that the biggest problem I had with it was a lack of flow and a weak protein skimmer.