How can I be sure that my child’s propionic acid levels have dropped?

The intestinal bacteria can communicate to the brain through 5 or 6 different pathways. In kids with autism, I like seeing signs that they are calming down (better sleep, less anxiety, less stemming) and more in touch with their surroundings (improved eye-to-eye contact, recognizing the arrival of a familiar person). These are signs to me that the propionic acid levels are dropping.