APT Secure is an alteration to APT to complete this chain of trust. With it, you can add a set of keys that you trust to your keyring. Then when you run a command like apt-get update, APT will walk back along the chain of trust for you, and freak out if something looks suspicious. (You, in turn will freak out, and do your best to freak out the next link in the chain.)
Furthermore, when you install a package, if the package cannot be verified, APT Secure freaks out, and suggests that you also freak out. You can choose to not freak out, and install the package anyway. We suggest that you panic.