Best way I know of before putting plants in is to flush the media with distilled water. If in a pot, plug the holes and let it sit in the water for a couple of hours. Then allow the water to drain into a cup to measure with strips or a pH meter.

If the pH is too high, acidifier can lower it, but the best way is to dilute it with more 5:1:1. Dolomite lime is a pH stabilizer, so may take a lot of acid to lower it. For the most part, dolomite lime will not usually raise the pH over 7, so a bit extra won't hurt most plants. Best pH for soil less though is around 6 for most plants.