Hmmm. I wonder what you'll ponder about for the next few years if this question is answered?
Laughter from tickling is an innate reaction. Being ticklish is theorized to have arisen to prevent us from dangerous creatures and bugs...hence the jerk away reaction from a soft touch. Perhaps the laughter is a result of the contradictory info to the brain...primal brain says "deadly creepy crawly on foot, get it off!" "modern brain says "that's just GQguy being silly by tickling my foot with a feather". Along with this is a loss of control. Some people like the loss of control...those are "ticklees". Some don't.
As for sandals. Just assume they are worn because wearing closed shoes during the summer is hot, and sandals are in style. Every once in a while you'll find a woman with a funky taste in shoes...it's most likely that she has a funky taste in style. Ticklees don't have a uniform unfortunately.
A woman in your bed is the most likely indicator of her being cool with you tickling her.
GQ