How do I distinguish between reliable and scam bounty?
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.
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Are there other ways to earn free digital currencies rather than faucet and bounty?
Money does not come for free in this world. 
NB: Topic should be moved to "Services Discussion (Altcoins)"