You need to get a drop checker and 4dkh solution.
You can make the solution yourself with bicarb soda. Just YouTube how to make 4dkh solution. You will need to buy the drop check tho.
You can also get a ruff idea by testing kh and ph and working it out on a co2 chart. (Google ph kh co2 reference)
Make sure you test the ph at the highest point of your co2 (just before co2 or lights turn off)
Thanks Bert.. added drop checker to the setup, will test ph tonight before turning off co2 and tomorrow morning before turning it on. Let see what reading says.