If its under Fedora (yum systems) you can do:
CODE
yum list installed | more
You can also do this with RPM (suse, mandrake, etc) with:
CODE
rpm -qa | more
To scroll through the "more" lists hit enter or space bar.
Not sure about slackware .. but I believe you can search and it will tell you if its installed or not.
CODE
swaret --search package
For debian systems:
CODE
dpkg --list | grep '^ii'
That should do it too.