There isn't any effective way to avoid scam projects, they are always available every time, here are few things to watch out for to atleast be more safer ..
1. Join bounties from reputable bounty managers.
2. Avoid bounties with high allocations
3. Join bounties with limited participants
but that is the thing, most of the hunters now doesn't care about that, they are just joining any project that they want, and doing task without effort I mean, I always see some of their posts from other campaign then they will copied that and put different hashtags then they will submit it, as of it is original ones. it's kind confusing that why don't they even do some effort to make a quality posts or even check the Bounty before enything else.