Many soapers want to absolutely know their soap has no excess lye so it is safe for use on the skin. There are several ways people commonly test their soap to determine whether there is excess lye.
Some test the pH with phenolphthalein drops, pH test strips, or even a pH meter. The drops and test strips are notoriously inaccurate -- I cannot recommend either of these for testing the pH. A pH meter can accurately measure the pH, but the meter must be properly maintained and calibrated and the test must be done correctly using a diluted soap solution.
Even with an accurate pH reading, a soaper must properly interpret the results. The pH for properly made, skin-safe soap can range from about 8.5 to about 11, depending on the fatty acids in the soap recipe. If you insist your soap, regardless of recipe, must always have a pH of about 8, you will be a very frustrated soaper.
The industry standard test for accurately determining whether a soap has excess lye or not is NOT a simple pH test. It is the "excess alkalinity test." This test is something a handcraft soaper can successfully do, but it is more involved and time consuming than most soapers want to deal with.
A quick, low cost, and acceptably accurate alternative is the "zap" test. Many soapers dismiss the zap test as being unscientific and unreliable, but history proves otherwise. Soap makers have used the zap test for centuries to make quality soap. A quick pH test might provide the illusion of greater scientific accuracy, but pH is not a replacement for the zap test nor for the more rigorous excess alkalinity test.
How to do the zap test
Wait to test any finished soap until two to three days after it is made to allow enough time for saponification to completely finish. There is no reason to zap test any quicker than this if you mainly want to double check the soap for safety.
New soapers -- this is warning is especially true for you! You are much more likely to make serious mistakes and be the least aware that you have made them. That lack of experience means you are much more likely to have lye-heavy soap. Please handle your new soap with gloves and wait a sensible time before zap testing (and bathing with) your new soap!
If you are a more experienced soap-making nerd like me who likes to do science-y experiments and is fully aware of the consequences of handling and zap testing a "hot" soap, then test at your own risk. You've been warned, so no whining!
When ready to zap test, dampen a fingertip, swipe it lightly on the soap, and very lightly touch the fingertip to your tongue.
If the soap is lye heavy, even slightly, you won't have to ask yourself whether you got a zap. It is not a taste; it is a sensation like a static shock that is unmistakable and immediate.
The intensity of the zap will vary. If there is a slight trace of excess lye, the sensation will be a mild tingle. If there is a fair amount of excess lye, the sensation can be a strong snap to a burn.
If you find yourself wondering or if it takes a few seconds before anything odd happens, you are probably reacting to the taste of the soap itself. Soap often tastes surprisingly bland, but it may have a metallic, bitter, sour, or unpleasant tang depending on the fats and additives. In particular, fragrances can taste icky, even though they might smell wonderful.
If there's any unpleasantness going on in your mouth, immediately spit, don't swallow, and rinse your mouth well with cool water until the unpleasantness is completely gone. Wash your fingertip well until any slick, syrupy feel on the skin is gone.
If you do not have any unpleasant reaction to a cautious zap test, then feel free to lick on your soap to your heart's content.
Oh, no, my soap is zappy! What do I do?
If your newly made soap appears to be fine but has a slight to moderate zap, do not be too quick to throw it out or rebatch it. Let it cure for several weeks and zap test again. Chances are very good the soap will be fine.
A small amount of excess lye will gradually dissipate during the cure, so simple patience is often the best (and easiest!) solution.