It is hard to tell if a bounty is a scam or not. It is still a real project after all. The only thing is that the team, or the manager will not pay the participants. Scams are quite impossible to be determine through facebook or twitter account neither its ANN and bounty thread. Those page or thread will be showed in public so they will not let that the public will notice that the project is a scam through this. Probably they will put different attractive things like its big allocation.