Answer first
To patch-test, put a small amount of the product behind your ear or on the inner forearm, leave it for 48 hours, and watch for redness, itching, burning or swelling. If you react, don't use it on your scalp. If nothing happens, start with the normal amount.
How
Clean skin, a small dab, no covering, 48 hours.
What a reaction looks like
Redness, itch, small bumps or swelling. Wash off and stop.
Questions
Is a patch test foolproof?
No, but it lowers the chance of a surprise.
Should I patch-test Folli-Fix?
If you have sensitive skin, yes.