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.
but what if the campaign manager that you followed is also a scammer ? you know most managers and reputable members on this forum can also do unexpected things and that is to scam thier participants by taking away the budget that is usuallly allocated for the campaign .
if you dont want to get scammed then dont ever try to join a bounty , instead join only activities that arent risky like for example working on a free lancing work online or apply for a regular job on a real company .