Phenolphthalein is a liquid that can be use to indicate your handmade soap is safe to use or not. Use 1-2 drop at a time, to soap. It turns pink when it touches a base. The darker the shade of pink, the higher the pH reading. You won't get an exact pH value, but you will be able to tell, quickly and easily, how far along the pH scale your soap is.
To use phenolphthalein, wait until your soap has completely cured (6-8 weeks for cold-process soap). Cut a bar of soap, and place a drop or two of phenolphthalein on the cut edge (so that you are testing the "inside" of the bar). Wait for the color of the drops to stabilize, and see what you have.
If it's clear or light pink, your soap is safe to use. If it's darker, proceed with caution! The darkest shades indicate that the soap is dangerously heavy in lye, and should not be used!