I guess that you are thinking about 'check-update' yum command. If there are updates then yum will produce a list of available updates and will exit with a status 100 and otherwise this status is 0. Once you have a list you can retrieve packages by whatever means you have on hands.
The pre-2.1 yum included the option to the update command to download updates but not install them. This eliminates the need to wait for them to download once I've blessed them. I'd use that option in the midnight cron job, then apply the updates in the morning. If all went well, I'd not waste time waiting for the download, and if there was an issue, users wouldn't have to wait for me to get in to fix it.