"ask-out-able"
LOL. I love non-existant, gramatically incorrect words like that.
To address your question: I'm not sure I have a specific answer. For me, it's probably very similar to how guys decide whether to approach a girl.
Most guys who I've asked out have been guys I've met at least one other time beforehand through other people, class, work, etc. There's nothing really consistent with those guys other than that they were smart, funny and showed a genuine interest in me.
For the guys I've never met before, they have to be well dressed. They don't have to have the best clothes money can buy, but they have to look like they take pride in themselves and their appearance. I have to find them attractive (of course). A smile doesn't hurt, and maybe a look or two in my direction.
If he's alone or with only one or two other guys, I'm more likely to approach him than if he were with a large group. I would find that intimidating. If he's with a group, he should come to me where we can talk more privately.

Also, if there's another woman around, I definitely won't approach unless it's clear that she's not with him (like if she kisses another guy or something).
If you're asking to figure out a way to get women to pick you up, I don't think you'll have much luck. Not a lot of women are willing to do that. They see it as the guy's role. So if a woman happens to approach you, it's just chance.