They sell test kits for this but they are very inaccurate.  The best way is to be sure that you are not using pH Up and pH Down or any other water additives that are phosphate based buffering agents.  If you are not using these then there is a simple chart that can be used comparing pH reading to degrees KH reading.  As long as you use carbonate as a buffer and are already adding CO2 to the water you will get a relatively accurate measurement of how much CO2 is held in your water.  Typically the suggested amount is somewhere in the 10-25 ppm (mg/l) range with 15-19 being optimal.

 For a estimation chart click here.