I've look through the Prefences box and the notification icon's context menu but found nowhere to see the current version of the application.
Do you see it elsewhere?
My OS: Windows 7
Hover your cursor over the Dropbox icon in the tray and a pop up should appear telling you the version of Dropbox you are running.
If you're on a Mac: Applications → Dropbox → Select it →
Edit: sorry, I did not see you are on Windows. I'll leave the answer, anyway.