How can I tell if I can use my subscription on more than 1 computer?
If you're running IS2012, you can check directly from Launchpad. Click on the menu below the F-Secure icon and select "View my subscriptions". You can see there how many computers of how many (1/1 or 2/3, for example) are installed and in use.
Other than that, with the older product versions, it's more difficult. It's listed on the mail you got when you bought the software.
The other way to find out is to give support a call and have your subscription number to hand. I've just confirmed they should be able to check it for you.