[orblivion]

If you trust the source code of the software you're running, you can at least get a sense of what data they're getting in the first place. You know, at least, that they're not getting the content of your communications if you verify safety numbers. You can also prove that they're not getting the contents of the gifs you're grabbing for your conversation, because the client makes a secure connection to the gif service using Signal's servers as a proxy.

As far as promising not to store your metadata, or promising not to deliberately give the gif service information about your account because they hate you, or promising not to store your contacts when you search for other friends with Signal, then yeah you have to just take their word for it. Though, they may over time look for ways to put some of those guarantees on the client side as well with some clever engineering, so you could prove it.