In short, none. The Flashback malware exhibits no symptoms.
The latest variants of the Flashback malware, which have infected more than a half million Macs, not only install invisibly, but also exhibit no symptoms. If you are infected by this malware, you will see no change in your Mac's activity. The only way you may see some activity is if the Flashback malware were to connect to a remote command and control server, and you were monitoring network activity, using a program such as Intego VirusBarrier X6.