It's all depends we choosing a project.. If you want a good quality bounty, just follow the most trusted bounty manager, find project who have a strict rules for participant (strict rules have possibility to avoid who cheat on bounty).
It is the best strategy to lessen the chances of falling into a trap but cannot be avoided in some cases because of scammers had also strategies on how to convince managers to work for them, as I 've said it cannot be avoided but at least it was minimal.