If I don't get a message, how do I know if malware has been installed?

Well, if you haven't noticed anything you are probably fine. As that support article (HT4650) indicates, Apple put out a security update that should remove it. Hopefully your Snow Leopard installation is fully up to date but you can run Software Update to make sure. If you still have questions, follow the instructions in the article to see if there are any relevant processes running, as viewed in Activity Monitor, and check your Applications folder for the malware.

If you don't get a message how do you know you got a message? 😕😉

If you mean a message without a body it may be someone searching for valid email addesses. If the sender doesn't get a bounce email message they know the email address is valid somewhere.

Seriously, you are probably fine but for peace of mind, I see from the support article describing what to expect in the Activity Monitor:

"Under the Process Name column, look for the name of the app and click to select it; common app names include: MacDefender, MacSecurity or MacProtector."

You need to make sure that "all processes" is selected in the menu at the top right of the Activity Monitor window.

I think you can sleep soundly now.