I agree with most of what has already been said. Being playful, especially with little pokes & tickles, and having that reciprocated is a sign to me of possibly more. Touches are also something I would look for or do...put an arm around her or something like that and notice her doing the same, or getting even closer would be a sign to me. I also think that wanting to spend time alone with each other is a good sign that there may be more interest there. I've had girls ask me if I wanted to go for dinner or to an activity/event...but if they ask if I want to bring a friend too I would tend to take that as a possible sign that they don't want it to seem like a date. If I'm interested in being more than friends, I also would be more likely to offer to pay for her dinner/drinks/movie/etc if I'm interested in her. I would also tend to look for signs of being more nurturing and really showing you care about each other...does she offer to cook or bake for me? Does she invite me to her place or always want to meet in public? Do other friends ask if we're dating or interested in more? Maybe they are seeing or hearing something that indicates a possible connection. Do you talk about each other's love lives and ask each other for advice about dating? It's probably not a good sign if they do, but that could also be an opportunity to ask what the other looks for in a potential and try to steer the conversation towards how you could meet those desires, or listen and act on that information in the future. I would also suggest upping the ante a bit with the flirting and see how she responds. I've been called out on flirting with female friends, but that gave me a good in to specifically ask her out on a date. Finally, compare how she acts with you to how she acts with other people of the opposite sex. If they seem pretty shy or reserved with everyone except you, I would take that as a good sign. Overall, listen and read their body language, and really...just carpe diem, seize the day in a way that you are comfortable with (without being afraid to stretch yourself a bit). Lots of people say they don't want to ruin the friendship, but I think that missing the opportunity for that special someone is even worse. If it's a strong enough friendship, the friendship should survive, and if not, c'est la vie. That's life. I think there are very few male/female friendships that can, or even should, take priority over a romantic relationship that leads to marriage.
None of these are hard and fast rules though...but I think seeing several of the above in combination means there's a good chance of there being interest in more.