My Account has a (1) next to it and the drop down says a game update is ready. Problem is I don't know which game. Any way to tell?

I am probably being dense, but I can't find it.

You have to browse your shelf and over the game which was updated there is a small red sign saying "update".

btw games which were updated recently were witcher 1+2 and dungeon keeper 2, perhaps it was one of these.

View your games in "List Mode" and you should see it right away.

Add to what has already been said by moonshineshadow & Licurg that, if only part of the content has been changed (new or updated extras), will appear a small sign with the word "NEW" in white over red background on the right of the new/updated content.