I've played this game a lot (though never on a forum) and have learned several things.
1. Default to rejection - At first, it may not seem it, but one of the best ways to figure out who is who is to reject teams and see who approves. The spies are going to be trying to get one of them on the team, so seeing strange approvals helps find them. I think it is MORE important here because we can't see each others' faces. So our only way of reading each other is from comments and votes.
2. Go until you get burned - One of the best methods is to keep as many elements of the team consistent until someone throws a fail. Example: Araver & I passed a mission. The next team would have been good if it had been me, Araver & 1 new person. That way you have two people who've passed one and only one new element. If a fail comes up, then you know there is a spy between the 3 of us. The only question then becomes, was it that new element or did one of the originals hold out? Even finding one spy early is a huge advantage for the Resistance.
3. Like Mafia, don't carry over - If you are a spy this game, you might not be a spy next game. Don't use that to figure people out.
4. Watch for tendencies - This is a lot harder online, since, as I said before, no reading people... Also why the votes become more important. If I see someone approving missions with a certain person on it, I can link those two in my mind. Then, I'll ask that person why they keep voting that way. Or why they keep putting people on the team.
That's all, I'll stop preaching now.