When you know someone is for you, is very subjective and varies from person to person.
Me, I know the moment I start talking to someone at the very least whether there's enough of a "click" to continue talking to him. He might not be "for me"; that isn't always possible to predict (though I knew it with Hubby and with my ex-boyfriend within an hour or so of the first "hi, nice to meet you"), but I at least know whether they're someone I'm interested in. I've learned to trust that initial instinct and walk away if I don't feel that connection, or if I feel uncomfortable, because the times I've ignored it haven't ended well.
If you don't feel the "vibe" with someone you meet, why agree to meet again? They might be a perfectly nice person, but in my opinion, life is too short to spend social time with someone you don't actually want to spend that time with.