How do I distinguish between reliable and scam bounty?
Project, find a project first then join bounty for that project. Don't go directly to Altcoin Bounty without reading Altcoin Announcement.
Are there other ways to earn free digital currencies rather than faucet and bounty?
Airdrops? Maybe not good but you will earn more than on faucet.
Unless someone donate you or send you a tip for useful post or help I don't think there is.