A bounty being a scam is from different levels :
1.The project is a scam - you need to do your own research about the project that you promote for bounty. If they turn out scam not only would you lose your money that you put your hard work in for but also your reputation for supporting them.
2.The manager is a scammer - in this case you have to prevent yourself from joining every new bounty that appears and go with those managed by trusted campaign managers.
3.Project fails to raise enough money - then they are not a scam but they failed in their objective. Nothing you can do to prevent it.
1.The project is a scam - how can we know if a project is scam ? , some project looks verry legit at first but eventually they are slowly turning into a scam .
2.The manager is a scammer - how to know if the manager is a scammer ? not all trusted and reputable managers are legit . some old members were verry trusted at first but eventually they did also turn into a scammer .
3.Project fails to raise enough money - how can we know that a project will fail ?
its really hard to predict the outcome of a bounty without trying them out first . bounties are like gambling both are verry risky . luck does matter the most on here
It is hard to predict if a bounty is going to be successful or not but it is not impossible. There are dozens of people who make profit constantly from bounties, and i dont think we can talk about luck here. There are different methods and specific informations you have to find in order to increase your chances of choosing a good bounty. This is something related to experience, because thats whats really going to help you to choose the best bounties. But now here the question, how can you have experience if you never invested in bounties? The conclusion is that bounties are not the best solution for most of you to invest your money.