If you do not have a pH meter I would follow JDs advice
I remember reading about this old 1900s (before pH meters were available) heated metal bar technique to check the acidity of curds during cheddaring. I think its called the "Hot Iron Test".
A more convential,fast and easy way to do it is check the whey's acidity.
It requires just a volumetric plastic syringe,clear glass\plastic tube, color indicator and 0.1N NaOH.
There are "wine acid test kits" which can be bought for about 10$ with the entire kit assembled.