To avoid a bounty scam, you need to find a bounty that has escrow for payment.
I have participated in bounties where managers escrow payment, but some of them ended with
1) receiving tokens that are not traded anywhere,
2) not receiving rewards, because manager hold tokens, but he was not given funds for distribution.
3) receiving tokens, but project rather quickly released new contract and refused to swap tokens.
And you can not deny the fact, that the manager can scam hunters also. What I would like to point - escrow does not guaranty success.