Look first if who are the bounty manager, trusted bounty manager will have you a guarantee that your bounty is not a scam, then check if the bounty has a potential to succeed and always check if the team always make an effort to have many developments in their project.
I think the best way to choose a bounty is to read their whitepaper and look for their team and manager. Find some additional info and study their website to see if its really legit and prductive.