I think you just have to know... I'm no expert, but I don't think you can check on installed apps in the shell like you can check on running services. You can see your directory structure... if you've installed apps like I used to in DOS you'll have each one in a different directory ("ls -l"). If you're running redhat, you can use the Redhat Package Manager to see what packages are installed too...
Right now nothing... but I was running Slack9, but I am gonna give Debian a shot. So far Slack9 has been my fav, but I really need some way of learning all this linux ****. And I cannot find a damned guide anywhere on the friggin internet! Everything costs money.
Kinda depends on how they were installed and what you are looking for. If they were installed as packages, you can use the package manager to view the installed packages. Other stuff you might have to look at the list of items installed with the original install. Things you compiled yourself, well you need a memory.
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As there are lots of Distro specific functions for things like that.
NS
http://www.slackware.com/book/index.php?source=x4000.html