New answer for newer versions of Ubuntu
I was searching online looking for this answer and suddenly found it myself. But in case anyone is still searching for an answer to this in a newer version of Ubuntu (ie. 11.11+) here's what I did.
Click the "Dash/Home" icon in Unity. Type in the search term "details". You will get a little gear icon with the word "details". This will open up a box showing you information about your system (sort of like right-clicking on the My Computer icon in Windoze and clicking Properties.)
Right on that first page near the bottom it should tell you "OS type".