"In order to do this, it cross-checks your Mac's unique hardware with its own database of machines that have been compromised," writes CNET's Josh Lowensohn. "If it doesn't find your machine, you're in the clear."
As Forbes' Adrian Kingsley-Hughes notes, a more automated tool called FlashbackChecker is also available on GitHub. "It WILL NOT remove Flashback, but it will tell you if you are infected or not," he writes. "If your system is clean, then as long as you’ve applied all the updates for your Mac, you’re safe."
"Last week, Dr. Web estimated that more than half a million Macs had been hit with Flashback through social engineering trickery and drive-by attacks that silently exploited a critical vulnerability in Oracle's Java," writes Computerworld's Gregg Keizer.
"The Trojan first appeared disguised as a Flash installer last September, and disabled Mac OS X’s built in malware protections," explains ExtremeTech's Ed Oswald. "This version makes its way into Macs through a Java vulnerability, and is loaded onto unpatched Macs without interaction from the user."
If either tool indicates that your Mac is infected, F-Secure has published removal instructions here.