you can get a chemical test for 5-6$ at a pet store or test stips or a meter at a hydro store. the best way to fix your ph is to start with the right ph by adding 1-2 tbsp dolimitic lime per gal soilmix when you make it. also use ph up and/or down to adjust the ph of the water and ferts to around 6.4 EVERY TIME YOU WATER/FEED THE PLANTS. if you do this you should not have any ph problems. i suppose if you already have a problem you could repot using the lime when you mix the soil. good luck