The most straightforward way is to ask her out. Make sure that she knows it's a date, and not simply two friends meeting for a drink. She'll either say yes or no.
If she says no, it doesn't mean she's not interested. She may be legitimately busy, or still not sure about you. Ask her again in a couple of weeks. If she turns you down again, move on. If you keep asking, you will likely creep her out.
When I was about 25, I was in this group of friends with this girl that I was interested in. When I realized that we were going down the "friends" track, I asked her out to dinner and a movie. She said yes, we had a nice time, and started dating. Now she's my wife.