When you click on the gear on the login screen, you see a list of all
installed desktop environments. It will clearly say Ubuntu or Unity 2D and
other choices if you have them installed (GNOME Shell and GNOME Classic or
GNOME Classic (no effects)). If you have installed Ubuntu and it shows but
does not work or defaults to GNOME Classic then you have either no 3D card
or an improper driver for compositing to work.