Linden Lab Official:Can I find out if someone I referred upgraded to a Premium account?

For privacy reasons, we can't tell you which of your friends have gone Premium. However, if you keep a close eye on your Linden dollar balance, you will see a L$2000 credit when one of your referred friends upgrades his account. If all else fails, you can always try the old-fashioned approach: ask them! When someone you refer signs up for a Second Life account, they automatically request your friendship when they first log in.