Kubuntu Lucid user here. When I run
sudo apt-get update I don't get a list of packages that need updating, I just get the list of the repositories being checked. For example:
Get:1 http://packages.medibuntu.org lucid Release.gpg [197B]
Hit http://it.archive.ubuntu.com lucid Release.gpg
Hit http://archive.canonical.com lucid Release.gpg
Get:2 http://packages.medibuntu.org lucid Release [6854B]
sudo apt-get upgrade gives me the list of packages that can be updated, if there are any. If you apt-get says there aren't any updated packages, I guess you can trust it, unless there's some problems with your
sources.list... But then apt-get update should tell you about them.
By the way, lately I've been using aptitude instead of apt-get: I find it better at managing problems, got me out of a tight corner once or twice... I just do:
sudo aptitude update
sudo aptitude safe-upgrade
I hope this helps.