to check soil Ph usually you can check your runoff or waste water (water from the bottom of the pot). to do this water the plant let it sit for about 10 mins then re water so you get a good cup or 2 of the runoff, then collect it. let the sediment settle for about 5-10 mins, then use a Ph tester to check the Ph of that runoff water (water from the bottom of the pot).
Usually everyone uses a Ph tester which is a drop solution and a little vial. there are different Ph testers with the best being digital, but the droppers is a standard, its what almost everyone uses, especially when starting the kit is about 6 dollars vs 100 for a digital Ph pen type.

pH Test Kit by General Hydroponics | pH & TDS Testing

more than likely you need to add some dolomite lime to you pots , but do a Ph test first, dolomite can be added to the topsoil so no need to dig up the plant. dolomite lime raises the Ph of the soil as most soil is low in Ph
MadSativa Reviewed by MadSativa on . Correct pH for soil... If your soil is either acid or alkaline, would you need to adjust your water/nute pH accordingly? For instance, my girls are suffering from lockout due to soil being slightly alkaline. In order for my runoff to come out at a pH of 6.5, I had to flush with plain water pH'd at 5.1. My question is, should I always feed/water with a pH of 5.1(+/- depending to run off)? Or do I continue to feed/water with a pH of 6.3-6.7? The goal is to have a soil runoff pH of 6.5 correct? I was Rating: 5