I've been on dates with guys from dating sites, two of my ex boyfriends and my current boyfriend I met on a dating site.
Honestly, the first ex I met on a dating site, he knew right away and asked to be with me on that first date. The second ex, we had been talking for three weeks before we met, we met up after that while because I was out of state for a couple weeks so once I got back home, we met up. Two days before we met, he asked me to be his girlfriend.
In my experience, only a couple guys I've talked to on dating sites ended up being with me. So it really depends I guess, you might be able to tell right away or after a while.