We have no real way to check for backdoors in Signal either. Signal is not transparent about what code their servers are running, and you are not allowed to start your own server with a known version. They do not allow for independent distribution of reproducible builds on F-droid, or any other application store that does not identify you. They will take steps to lock out any independent implementations of the client from their servers. That the code for their client is released is good, but not good enough.