Well it's very hard to know if a project is a scam or not. Also, some project are not scams but will still fail, because they were to ambitious, or their marketing wasn't effective, or simply because other projects were better then them. These projects will also fail, and some people will even call them scams, although they weren't.
As for your question, when a project completely "smells" like a scam (like a ponzi scheme for example) I will simply not do bounties for them, and most users in the community won't do it either, because this would make them scammers as well, and earn them negative trust. What kind of scam projects have you seen in this forum, that were supported by it's users?