I think the surest way is to look at the bounty Manager. If he has some experience and reputation in the community then most likely the new bounty company will be good. IMHO
Not 100%, there are bounties who hired known bounty manager but turned out to be a scam. We can see this kind of stuff in reputation and scam accusation boards. It is still a gamble and there is no 100% sure in this kind of industry.
for now, it's very rare that we find a good bounty, maybe this happens because there are bounty popping up, my advice is to choose a bounty with a clear and real website and concept.
I agree that it is rare to find a good bounty campaign but I disagree on just basing the decision on the neatness of website and concept. There are lots of ICO with these kind of website but turned out to be a scam at the end.