Effective network sniffer detection tools?

The SDBot.UJ worm contains multiple threats, including: a network-sniffer, key-stroke logger, denial of service, and backdoor capabilities. The worm also attempts to steal CD keys for several computer games.

Read more about the SDBot.UJ worm:
http://www.virusthreatcenter.com/permalink.aspx?BlogId=36

While all these threats are serious the inclusion of a network sniffer is of special interest. The carnivore sniffer filters for a list of strings, including several PayPal related ones. Which cause me to ask, “What are the most effective tools for detecting malicious network sniffers?”