Whey ph is .1-.3 higher than curd, generally. Depends on the milk. And the difference becomes bigger the lower the pH. With mozz, you have to test the curd, unless you standardize the milk because it's all about the texture. I can get a great mozz at 4.9, and a great mozz at 5.4, depending on the milk and whey drain pH (suggest 6.0) and rennet pH (suggest 6.3). And the point when to stretch is all about the feel of the curds.
No need to use a large piece, just cut off a sliver and stretch it in hot water and see what it does.