Verify Cyanogenmod update - checksum?
You dont have to worry about corruption of update. If its corrupt, the zip file will not be parsed and recovery wont flash it, no damage done.

Still, if you want checksums, you can go to http://download.cyanogenmod.com/, find your device and check the checksum of latest build for your device.

If you are still not comfortable installing update, backup first using recovery. You can always restore if it doesn't work out.


  Are updates signed?


I dont get what you meant by that. CM updates are signed with Android's default debugging keys (test-keys). Ofcourse feel free to research about this on your own at Cyanogenmod Wiki.