Candy, I have a digital pH meter that I use on my plating solutions. There are has 3 solutions of different pH that I used to calibrate it with. I did it once, when I got it last year and haven't since then. But I don't use it much. And never used it on soap.
It has 2 or 3 screws that you turnn on to calibrate it, while it is dipped in a calibration solution. If you just want to check it accuracy, dip in in the solutions and see what it reads. If it's off, just turn on the screw for that solution. Rinse with distilled water when going from one solution to the other so you don't drag any solution to the next bottle.
Spend your money on oils Candy. If your soap don't eat yer skin off, it must be good.