Follow credible bounty managers. Mostly, startups rely on bounty managers with enough experience in marketing and handling of bounty hunters. You can check them out via telegram and bitcointalk. Been tailing some bounty managers and teams who has handled great projects.
I have seen a lot of credible bounty managers here where in the end they've became greed and abuse their popularity here in the forum, while doing some activity which is illegal like they run away the fund of the owners of the project they managed then some are doing cheating abuse, or etc. Meaning, credible bounty managers is not enough to say that the bounty is credible.
I think as long as the bounty team of the project is transparent to their community, the the project is running in good progress, and always giving an updates to all socials media they have I could say that campaign bounty is credible.