it saves time to ask first
I tend to ask the girl upfront. I don't have time to take her out for a few dates just to find out she was in a LTR the whole time.
If you ask and she is looking for someone, she'll say no. You can read the last sentence again. Even if she has a boyfriend but is interested she'll say she doesn't have one. So the only thing you'll achieve by asking is to filter out some of the ones that will waste your resourses.
As long as she considers it even slightly possible that something develops between you, she'll say no. And, if 10 minutes into the convo, she still doesn't feel the slightest attraction for you, the best thing you can do is walk away. There are TONS of girls who would love to go out with you and have a great time without even thinking of putting out. Keep in mind that asking her in the first convo will filter out only SOME of these.
There will always be chicks who go out with you for the expensive dinner, or for the free amusement park tickets, or because her friend is away for the weekend and she has nothing better to do. not to mention how much more often you'll be getting stood up because the chick didn't find a way to make it clear she's not interested.
Oh, and make sure you weave it naturally into the convo. You don't want to scare her away by putting it too bluntly, which shows poor social skills.
Don't be shy and ask her. With one question you subtly show your intentions and save time and energy. It also shows her you're the kind of guy who likes to know what game everyone is playing. You'd be surprised how many AFC's spend dozens of hours chatiing up and entertaining chicks without ever realizing they didn't stand a chance in the first place.
I always feel women respect me more after asking that question too, especially the ones that say they're unavailable and for one reason or another we have to meet periodically.