It's lame to say "myself," but it's true that the intensity of my own acute vulnerability to tickling is the index by which I recognize excessive ticklishness in others. There was a girl in high school, a lanky redhead, who was an inveterate tickler; she victimized me, of course, but I wasn't her only target, and I remember the exaggerated convulsions and instantaneous begging that would ensue whenever she went after a girl named Stephanie (which, needless to say, she did with regularity). Stephanie was near-phobic about getting tickled, probably thanks in part to our classmate's merciless attentions, and if she was in the mood to be paranoid she'd flinch when a hand came near.
Meanwhile, I used to work at an office where there was another habitual tickler, Jen; again, she made a habit of fluttering her fingers up my sides and enjoying the involuntary dance of agitation that ensued. But she also identified another coworker, Amy, as vulnerable, and again, the extremity of Amy's susceptibility to tickling was evident: she didn't want to be tickled and she especially didn't want to lose control in the workplace, so whenever Jen goosed her sides or poked her tummy or squeezed her kneecap you could see the strenuous effort she was putting into not flinching or flailing too much, and the helpless rictus grin that would spread unbidden across her face as she desperately tried not to giggle audibly.
Both of these women were probably among the more ticklish people I've met, and I feel like I can say that with confidence because everything about the physicality they exhibited when tickled spoke directly to my experience -- I've felt what they were feeling; my body betrays me in the exact same way.
