Since people with AF tend to be hyper-reactive to certain supplements, how do you make the distinction between having a reaction to a supplement and just having a bad day symptom wise?

Allergies to supplements can manifest in various ways such as hives, itching, swelling of the tongue, wheezing, etc. Another simple indicator is a rise in the pulse. This is from the epinephrine released from the adrenals during an allergic reaction in order to try and counter the allergic reaction.

If they do not have severe allergies you can also have them try a small amount again on another day to see if they react again.