Well you can do a bit of research too - google image search, Facebook, LinkedIn etc.
Sometimes your gut instinct will be right, sometimes it'll be wrong. But the consequences of ignoring a bad feeling about someone are potentially worse than the consequences of heeding it.
queen - well someone who's former forces with no friends, that's maybe an amber flag in and of itself, not a red one.
However someone who just wants you to come round to his, and doesn't understand why you may be reluctant, that's a reddish flag. Combined with the other, I'd say this is one to avoid. A Facebook profile can be faked. Trust your gut on this one.
lovemusic - I think you should walk away from MrSurf. You're flogging a dead horse here. It's been how long? He's not pushing the right buttons, or you're not pushing the right buttons for him. And you're not even sure he's not married yet?
Not all men are bad. I'm not, and I'm not particularly unusual, I think.
The trick is to know when to give up on something which isn't making you happy, to work out if it's because of you or them or a combination of both, and to have a thick enough skin and ability to learn that you can go back and search for someone who's a better match for you