Most, if not all, anti-virus programs recognize the EICAR test signature. A similar signature is also avaliable for ClamAV, but this one is not recognized by any other scanners. To all intents and purposes, EICAR is all you need.
Go to http://www.eicar.org/anti_virus_test_file.htm and download the Eicar test virus, which is just a 68-character plain text file.
Email this file to your mail server that you want to test and look at the system logs to see what MailScanner does. Note that you can just telnet to your machine and send this string in the body, see the connexion page.
Most anti-virus scanners that are worth anything should pick up Eicar, and you should see MailScanner and your AV code noting that the mail message was infected with a virus.
You should also check to see if the message got cleaned and delivered, quarantined, or deleted according to your MailScanner configuration settings. The sender of the virus should receive notification that they sent a virus, if you have MailScanner configured to note infections.
Eicar should _NOT_ be in your Silent Virus list.
(Text by Jeff A. Earickson)
This signature is available in the ClamAV source package. Get it from http://www.clamav.net. Can be useful to verify ClamAV installs.