Yeah ... now it is very difficult to find a bounty that will not be a scam. But the plus is that you do not lose anything except your time - therefore you should not spend a lot of time on this, just consider it as a hobby.
Well, to find a good reward you need to participate in many bounty projects. To participate in many projects you need to spend a lot of your time. you just need to learn how to understand projects and then you can spend your time efficiently