You check the water before you feed, you can check the run off to find out the ph of the soil, or get a soil ph kit. If you want to change the ph of the soil use water with a higher or lower ph but not by too much as you want to do it steadily.
Liming Recommendations:
To make the soil more alkaline by 1ph apply 300 grams per square metre of Ground Limestone. To make your soil more acid by 1ph apply 70 grams per square metre of Sulphate of Ammonia.
Search around and find a good ph meter. While you wait though run to wally world and grab a test kit for pools.. They arent the most accurate way to get the ph but damn sure better than nothing.