It's hard to tell which one is scam bounty, which is not, even experienced bounty hunters are sometimes cheated. My method is actually very simple, just follow a bounty manager and only participate in the bounty campaigns he posted.
Choosing a bounty manager is a good solution, their credibility with the ICO project will be very familiar. So if in the middle of the bounty road is caught a scam, then the bounty manager will notify or even give a reward with his own money. And also, being active in scam accusations is a good act, by doing that you can see some people making threads with the intention of telling someone that there is an ICO that is a scam, from here you can do your own research.