You can't tell for sure. All you can do is go on dates, and hold things off until you feel you can trust him. When you feel you can trust him, it's your call. Same goes for sex. You hold off on sex, until you are at a place where you think he's actually there for you, not just sex.
This is strictly a judgment call, and nothing else. There is no set time limit. No set number of dates. No signs you can look for. No tell tale signs, logic or anything you can trust for sure. If there was a rule book, players would follow it to the letter, and fake all the signs. It's all up to you. This is with internet dating, or real life dating. It's all up to you. Nothing else.
You have to decide when you're ready to take the risk, because believe me, there's always a risk. (Also, while nobody ever talks about it, the same risks apply to guys to.) The risk is always there, that you might get your heart broken. You just have to be brave at some point. It sucks but that's just the way it is.
It depends upon how long the dates were and how much of the other person's personality and behavior was discussed and observed. You might get instant chemistry with a good match, but sometimes some people take longer to get to know, as they don't open up as quickly, but they still might be a good person. If both parties continue to want to see each other, I'm in favor of taking the long approach instead of the rushed one.
That is SOOO hard to determine because everyone is so different... some people are very genuine and decent and others are very good at hiding who they are... at the beginning they are on their best behavior, and a few months into the relationship all of a sudden you find yourself with someone you don't know! That's why you have to take your time and get to know a person. Sooner or later they show themselves.
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.
If you don't know by now, odds are they aren't. I can tell right away. About 15 minutes into the first date, I've already decided if there'll be a second.
that's a good question. I'm in the same situation -.-