You need to analyze the bounty as a whole. Check the team profiles, the project legality, whitepaper, their social media, and etc. If you see something strange or nonsense on them, you are better to avoid joining the bounty. Also, you are better to join a bounty from trusted bounty managers. Good bounty managers won't risk their reputation by handling bad bounties. Even if no guarantee for successful bounties, but they should have done some analysis before deciding to manage the bounties.