1. Escrow the bounty funds and make sure it's managed by trusted managers.
2. Never join any non-escrowed bounties if you fear the bounty won't paid you in the future.
Those 2 suggestions I provide above 100% will prevent from bounty scam and make bounty scam will go away since no one is interested.
These are probably the best remedies to make sure that bounty hunters will be paid for their work but it will not guarantee if the project is a real deal. Bounty hunting is dying, scammers only flock this forum for free promotions. I used to joined a lot of bounties before during when the payment was really good until it became worse. There should be a reform in this, to stop abusers.