Clam Antivirus features fast scanning and is equipped to detect over 1 million viruses, worms, and trojans. It also features support for most mail file formats and HTML.

The best (and perhaps usual) answer is to use an antivirus program.

For Windows, there are hoards of antivirus programs available like McAfee, Norton, Kaspersky, AVG, K7, Quickheal, Avast!, etc.

For Mac and Linux, there one AV software available called Clam AV (ClamAV.net).

You will see a clam.

a clam

It is a clam.