Some projects don't start as scam or they were too good in disguising themselves as legitimate projects while deceiving the bounty manager and her promoters. In case like this, you can't blame the bounty manager for not figuring things out in time but when a BM can be blamed and punished is when they have exposes the scam but yet they still associate themselves with the scam by managing their publicity.
I think it is quite tough to find out at the beginning. But for me, a scam project always starts out as a scam. If you have no intention whatsoever of actually doing work, developing product or solution, and just want to raise money, it is pretty obvious. Founders and devs who are hard at work need only show us github and all the stuff happening. If you only see marketing and conference and PR, for me ,,, that is a start of a scam.