Am I Currently Using WPMU?

Is there a way to see if I am using WPMU in any way? A Web Developer I hired a year ago was looking into this but went with InfiniteWP instead. Need to know as my sites are being moved to a Reseller acct in 12 hours. What do I currently get with my WPMU license that is better than InfiniteWP or what are the most common uses for WPMU that I can benefit from? Thanks!