I'm not sure what you are asking there.
rpm can sign and check signatures of packages.
yum can use rpm to check package signatures, it has no ability to sign them.
I was talking about koji (the fedora build system). It's been proposed that it will sign all packages it builds with a 'this was built by the fedora build system' and that would be used to check rawhide packages.
Currently there is no easy way to sign all the rawhide packages, so they are not signed.