apt-get -s upgrade
This lets me know when updates are available: the
-soption lists available updates without running them unattended.
Occasionally I'll get a list saying that some updates have been "kept back." I always have trouble remembering what to do in this case. It's typically just some dependency problem. This issue is addressed in the APT HOWTO on the Debian Web site. In my (limited) experience, this has always been overcome by doing
apt-get instal pgkname, where pkgname is the offending package which is being "kept back."