With the GPL, for any binary that you distribute, you have to make the corresponding source code available. With reproducible builds (see: https://reproducible-builds.org/
), it is possible to verify that the binary was indeed compiled from the available source code. All you have to do is to compile the source code yourself, and compare the resulting binary with the one distributed. For an example of this in practice, see guix's `challange` command.
With the AGPL, if you run a service accessible through a network, you have to make the corresponding source code to the service available to the users. But is there a way for the user to verify that the service provided corresponds to the source code available? I can't think of any situation where the service couldn't just simply lie about what it is.