How can you know which bounties are good? Simply read the coin/token's whitepaper. No one is going to spoonfeed you here, telling you which ones are good and which ones are not. At some point you'd have to do your own research.
Sometimes even how well detailed and properly managed the whitepaper is, still not an assurance that a project is good. For projects to attract investors, it's a must that they should have a good and stunning whitepaper with interesting roadmaps.
To OP, yes research is a must as we can't tell you honestly how to know if the project is scam because of what I said above.
Now to minimize the risks about your concern, then try to read other people's feedback on that certain project since you don't have an idea where should your start. You may look at their ANN thread here in bitcoin or try to locate all their identity at any platforms available on the internet.
All I can say is goodluck and once you joined their bounty, you must also accept that you are facing the risks of being not paid.
