I’m not quite sure what I’m specifically meant to be looking for to verify the safety of an application. Would you be able to elaborate? Apologies in advance, I’m very much a noob when it comes to the Linux ecosystem. 
In case verifying applications that way is just beyond my ability, I’d still be interested in knowing if there are any specific measures the Flathub team takes to ensure the security and legitimacy of their apps. For example, is there any vetting of contributors or the applications they build? Are there any automated checks which verify the legitimacy of an application? Etc. My thinking is that I’m already trusting Flathub to some degree, so if they take sufficient precautions to protect their users from potentially modified unofficial builds, I’d feel a lot better with trusting Flathub with important applications like Bitwarden and Standard Notes.