How do I tell if the Flash Player Installer is a virus?
There are a couple easy ways to identify the trojan in it's current form (aside from downloading directly from Adobe or using AV software):


The trojan is an installer package that opens with Installer.app, the real installer is an application (it doesn't use Installer.app). 
When run, the trojan installer looks like the screenshot of the trojan installer in the CNET article you linked to, the real installer looks like the screenshot of the real installer in that same article. (Screenshots reposted below.)


The trojan is found exclusively on sites that are not adobe.com, generally from malicious sites.

Legitimate Flash Player Installer:


Trojan Installer: