First, try to get the pH of your water as close to 7.0 as possible. Then water the soil until you begin to produce runoff at the bottom of the pot. Collect that water and test the pH of it. That's your soil pH.

Testing it now is fine, just so you know what your medium does to the pH of your water... but don't do it any more frequently than you need to. Especially while your plants are that young. Too much moisture can cause root rot, damping off, slow/stunted growth, and other nasty problems.