Easy to say that but in reality, it stinks.
Unfortunately, but you can not protect 100% against scam. In my opinion, there are only two things you can do to reduce the risk of being cheated:
1. Choose only campaigns run by trusted managers (they should verify the credibility of the project before cooperating with it).
Even they verify, it doesn't guarantee that they won't get scam as even managers are also the victim unless they are paid in advance right before they launch the bounty.
2. You need to verify by yourself if the project is credible by checking the dev team, whitepaper, roadmap, etc.
Some projects made everything perfect so we won't doubt and besides paying some bucks to professional web designer and content writer will be nothing on the amount that they can run of.