How can I tell if ClamAv is running?

ClamAV is designed to be an on-demand scanner, and will only run when you invoke it to run (which mean you probably won't see any of its processes if you did not manually invoke it). Also, when you installed the clamav package, you only installed the command-line scanner and scanning engine.

If you want to install the full package of ClamAV, I suggest you use this

sudo aptitude install clamav clamav-daemon clamav-freshclam clamtk

A breakdown of what package does what :

• clamav - the command-line based ClamAV virus scanner and engine
• clamav-daemon - enables ClamAV to be an on-access scanner, which means it runs automatically without your intervention
• clamav-freshclam - enables automatic updates for ClamAV
• clamtk - the GUI frontend for ClamAV

With all these packages installed, ClamAV should perform like most other AV packages. Like alex said, once you installed these packages, running ps should allow you to see the ClamAV daemon running.