I think the trusted bounty is when they have good management about the weekly report, when they always routine review the report and show the report, I think they have good management, so it always gives me trust to them.
Weekly report is a good one, it means the team is really serious about the project and always update the investor about the project report. To choose a good and trusted bounties, you can check by how many ETHs collected on the ICO. Good project usually have a good result on ICO.