The best thing is to choose bounties that are being managed by well experienced, seasoned bounty managers who have good track record in selecting good bounties.
I pay attention to the amount of money raised, which pool was allocated for bountyhunters and how long the campaign will last, because I do not like to participate in long-term projects with dubious results. But now even good projects at first glance may end up being a scam, so from the very beginning you shouldnt expect much, then results can be a pleasant surprise.