Looks like you might be able to use the combat log
http://www.wowinterface.com/forums/s...ad.php?t=34248
From 2010, but the idea is to scan the combat log for attacks that have you as the destination.
It wouldn't tell you about units that have aggroed on you but haven't attacked you, however. If you're in a party you can probably do as that thread says and also scan for enemies attacking party members, since that would normally put you in combat with them as well.