A lot of it depends on how long you've known the person and if you know they have any interests in anyone (ie, they have a significant other).
Most of the time, a simple, "Hey, want to go to lunch with me sometime?" is a good way to get to know them better and figure out if they're actually worth dating. I've done this a few times and the results were mixed, but that's why you do sort of like a pre-date date to test the waters.
If it's someone you know, or more specifically, know well, approach it kind of like, "Hey, I've liked you for a bit and I was wondering if maybe you'd be interested in being a little more than friends."
If you've gotten the, "You're my best friend/You're like a brother/sister to me." lines, you're dead in the water, and stop now. I tried to go out with my best female friend and it was nothing but torment for a long, long time. She said no, but it's hard to let go of something like that if you're really close and whatnot.