Personally, this is what I do while opting for signature bounties (the only I do).

1st. Evaluate a bounty program as if you are evaluating there ICO. Look for a solid project who has high chances of getting a successful ICO, this in turn give you the confidence that your effort are not going to waste.

2nd. Look towards a bounty which when rewards you give you tokens which are much worth on future exchange listings, dont look out for quick flip bounties, eventually, those are usually dumped during the listing.

3rd. Not always, but I prefer working under experienced bounty managers/companies like Amazix, this is my 3rd bounty under them and I am happy with the way they operate. I have done other bounties with few other good bounty managers as well like TKY which rewarded me nearly a BTC as in bounty reward.

4th. Dont FOMO, wait till you get a good bounty, dont lock yourself in any bounty. Its Ok if you are loosing couple of days or even weeks, but a good bounty program will instead give you better results.

5th. Currently I joined SpringRole (reasons I joined):
a. There hardcap is low which makes them an idle ICO
b. Managed by Amazix, hence I dont need to worry about my payments post bounty gets over
c. The Project is partnered with amazing companies like CVC, Science Blockchain, etc. I have personally checked every single partnership and found them genuine, all partners have a medium blogpost announcing there partnership with SpringRole.