I was told conflicting things by my local dealer (and yes, that is a witty drug parallel, haha)
Generally lace rocks (or lava rocks) are ok to use in a fish tank. However, if you want to test them, you can put a drop of vinegar on it and if it fizzes then yes it's going to change your pH. Another test would be to put one in a bucket of water and test the pH... wait a week and test again. If the pH has dropped more then a points then yeah, it's gonna effect your pH.