To choose a bounty that is legitimate you must first check out the bounty manager and his track record. Also you must check the team behind the project to be certain that they're not scammers. Also read the whitepaper to grasp the idea behind the project, and check if it's innovative enough to attract investors.
To know the income you could get from participating in the bounty, check the bounty pool and the percentage allocation for the specific campaign you want to do whether social media, media or signature and other campaigns.