Here's a little advice, if I can be so presumptuous to offer some. I would look for an opportunity to comment on her feet. Like, "Hey I like those shoes", or "I see you painted your toenails", "...You've got cute feet". I think that those of us that have this fetish always over-estimate the "stigma" associated with what would be a very innocent comment in most people's minds. I think that because we happen to be very attracted to and/or obsessed with feet, we are afraid to refer to them, or talk about them in casual conversation for fear that we will be labeled a "weirdo". I still get a twinge of embarrassment about saying the word "tickle" in a public setting. It took me a long time to realize that as long as you aren't constantly talking about feet or tickling, no one will even notice a casual comment here or there. She will probably be flattered and maybe a little shy about it. Just make sure you act like your comment was just in passing, not like it was something that you had rehearsed and conjured in your mind. If she gives you an opening, you can ask to get a closer look. Make something up about whether her second toe is longer (or shorter) than her big toe. You might get a chance to touch her foot and give yourself an opportunity to ask if she is ticklish. If she makes a comment like asking "...are you into feet", you can brush it off saying something like, "I try to notice everything about a girl. Why would you paint your toenails like that if you don't want anyone to notice?"
If you happen to catch her with her foot up and you can walk by it, go ahead and give it a quick tickle. If she is ticklish and flinches, you can give her the old "Ah your ticklish." and then try to tickle her some more and see how that goes. It might be an opportunity to wrap her ankles up under your arm and really give her a good tickle. Just remember unless you know she is really enjoying the tickling, don't press your luck and tickle her too long. You want it to seem like your just having fun and that it's no big deal. You might want to leave her with a comment like "Boy! That was fun!", and "...I now know your weakness", setting yourself up for a time in the future. The hardest thing for you will be to avoid tickling her the next few times you see her. If you try to get her everytime she's around you will be labeled as "odd". Layoff for awhile before you go try to tickle her again.
These are some situations that have worked for me over the years. Unfortunately, when I was younger and the opportunities were more abundant, I felt like a total freak about my fetish. I just knew I was the only one. It wasn't until I got into college, and after, that I took advantage of some of those opportunities. I wish I would've had the internet when I was younger. I, like many people on this forum, was suprised to find out that there are many of us out there. I still remember 10 years ago looking through a book at Barnes & Noble about the internet and seeing that there was a Usenet newsgroup called alt.fetish.feet and one called alt.fetish.tickling. I was setting up a Usenet account the next day. I can truly say that places like this forum have definitely made me more comfortable in my own skin about my preferences for feet and tickling. I've been married for 13 years to a beautiful woman, with exquisite feet, who will let me rub her feet and tickle her good from time to time. So in closing, I would say, proceed with caution, but don't keep your desires completely bottle up. The more girls you tickle, the better your chances of finding a girl that will enjoy it as much as you do.
Anyway, best of luck and good tickling.
Gallodiablo