How to check when yum update was last run

Solution 1:

The yum history option allows the user to view what has happened in past transactions. To make it more simple you can grep Update from yum history

# yum  history
Loaded plugins: fastestmirror, refresh-packagekit
ID     | Login user               | Date and time    | Action(s)      | Altered
-------------------------------------------------------------------------------
    41 | root <root>              | 2012-04-27 20:17 | Install        |   19   
    40 | root <root>              | 2011-11-20 10:09 | Install        |   10   
    39 | root <root>              | 2011-11-20 08:14 | Install        |    1 E<
    38 | root <root>              | 2011-11-19 15:46 | Update         |    1 

Solution 2:

I think the only way you can be absolutely sure is by running psacct.

This will allow you to run lastcomm yum. If you parse this, you will know who ran it and when.