In order to avoid bounty scams you need to consider 2 things:
1. CHECK THE PROJECT - you need to do research about the bounty, the project team, the whitepaper because some might just copy and paste it, the advisors, roadmap, bounty pool and ofcourse the founder you need to be a good observer, check their background because sometimes by just merely observing and reading you will know right away if the bounty is a scam.
2. CAMPAIGN MANAGER - the one that you should always consider, because if the bounty manager is scammer then the bounties itself is a scam, and it is really obvious if the bounty manager has a negative trust then surely he or she is scammer, one thing to consider also is the rank of the bounty manager the higher the rank the higher the legit factor, you can always do research here there are many list of legit and reliable bounty manager and the list of all bounty manager scammers.
This is absolutely the right way to do before participating on any bounty/signature campaigns but most people here will ignore this because of greediness to earn more. All they want is fast and easy money but they don't want to spend time to do research about the company/projects that they want to work with, and most of these people end up earning nothing because they didn't get paid for the reason that the project is a scam. Campaign manger is also a factor, most ICOs or companies that are being managed by reputable managers here in the forum are those legit ones, but I'm not saying that if the campaign manager is reputable you should not research the ICO/company that you want to participate with. It's still your obligation to know everything about them.