It will help you get started well if you buy a "drop checker", search in ebay for "CO2 checker". There are several different shapes, but all of them work the same. It is best if you get some 4 dKH water, made for this use, from someone here on the For Sale forum. Then, add that to the bulb of the drop checker and add a couple drops of the blue pH test kit reagent - like that which comes with the API pH test kit. The solution should then be blue. Add the drop checker in the tank so the bell mouth is facing down, trapping air in it. Wait a few hours, and the solution will be green to yellow green if you have a good amount of CO2 in the water. If it remains blue or blue green, increase the CO2 bubble rate. If it gets real yellow, you will probably need to reduce the CO2 bubble rate to avoid harming the fish. This isn't an exact way to measure CO2, but there is no exact way without spending $1000's for a test probe.