All personalities are given a pair of esoteric stimuli. Through reinforcement/punishment, one personality is conditioned to have a positive physiological reaction to Stimulus A and a negative physiological reaction stimulus B. The other personality is given the converse.

The stimuli are all drawn from a common pool of images like "bear", "hat" or "bicycle", so one half of a stimuli pair may be "a bear in a hat on a bicycle". There's a canonical set of stimuli, like a huge deck of cards, with all possible combinations, all of which are numbered. The numbers for my stimuli pair are tattoed on my body in some obscure location, like the sole of my foot.

If I need to prove my identity, I show my tattoo to the authority figure. It will read something like "1184/0346". They pick out either image 1184 (bear in a hat on a bicycle) or image 0346 (a sword in a hill being struck by lightning), and show it to me. My immediate response will be either arousal or disgust, and they will know which personality I am.