First and foremost I see how many coins they allocate to the bounty, to determine if they are paying a fair amount to the people they want promoting their product. Generally 1% is acceptable, and I've seen it range up to 6% which is phenomenal. On average the good ones pay out around 2%.
Next I look at the bounty manager, I see if they're established and if I've worked with them before. Good bounty managers are very important to me because I want to know if I have questions they'll work with me and communicate with me. It's also good to know your manager and if the projects they select generally pay out more than they don't.
After that, I check to see how the percentages are distributed and which media promotions are going to pay out the best. It's important to check the participation here as well, the more people in a bounty the less overall you tend to get.
After all this I look into the project and see if it looks blatantly scammy or not. It's really hard to tell though, generally if it already meets the above criteria I'll go with it even if it looks horrible on the surface.