I thought my pedicure idea was pretty good. I get pedicures regularly and as if that's not enough, after my pedicure I get to play with HER feet! (We've been friends for a long time and we both love the pleasures of tickling, of course).
Anyway... you want inconspicuous? In other words, you wanna find out for yourself if you're ticklish... with no interaction from other people, even a pedicurist? This'll be tough. I guess you could swab sugar water all over your body and then lie down outside and let the flies crawl all over you. That's bound to tickle, but it might be gross, especially if the fly just had lunch at Fido's Poop N' Scoop diner. Use sanitary flies.
I'm joking, of course.
Honestly, without any interaction from others I'm not sure what to tell you. Again, the pedicurist doesn't need to know that you're trying to figure out if you're ticklish or not. The moment she touches your feet (the light touch) and then uses that foot file or pumice stone on 'em (the firmer touch) you'll know at the end of the procedure just how ticklish you are.