The latest Steam beta update should keep all your DLC in order with its new interface.
Available now, the beta client update adds an improved UI to show what DLC is owned in the games library which should make it easier to see what DLC is actually owned without having to hunt it down. To see this new feature you will of course have to opt in to the Steam beta client but you can of course wait until this finally gets released on the live client.
Other fixes in this beta version include a crash in OpenGL games when taking a shot or recording a video, games list now shows recently purchased/played, ability to delete cookies in the built in steam browser, a hang on launch, and fixing changing between push to talk vs. voice activated microphone recording during a broadcast.
