Type into terminal:
unity --version
Hopefully that's exactly what you want.
Alternatively, click on the Applications tab in the Unity Launcher and search "Synaptic". You should see "Synaptic Package Manager", open it, and then search "Unity" in the search bar (or Quick Filter, as it's called). Your version should be listed underneath the "Installed Version" column.
I use a script to switch between Metacity and Unity and another to reload unity when I need to.
A probable solution would involve:
#!/bin/sh
#stop Compiz and thus Unity.
killall compiz &
#make sure to kill any conflicting processes (just in case, but this is optional).
killall metacity &
#This does not always exit cleanly, so kill it with fire for surety that it is dead.
killall unity-panel-service &
#load Unity 2D.
metacity --replace&
unity-2d-shell &
unity-2d-panel &
disown
^-- to load Unity 2D, and:
#!/bin/sh
killall unity-2d-panel &
killall unity-2d-shell &
killall unity-2d-spread &
killall metacity &
unity &
disown
^-- to load Unity 3D
You could save them as scripts in your ~/bin folder, for example as unity2dloader
and unity3dloader
respectively, and then simply run them from the run dialogue (alt+f2).
You could also place them in your nautilus scripts folder, and run them from the Desktop by right clicking on the Desktop, choosing the scripts menu and then the appropriate script.
Best Answer
Note:
As of Ubuntu 12.10, Unity 2D is no longer developed and all systems use Unity 3D (with LLVMpipe for systems without hardware acceleration).
The easiest way I have found is to look at the launcher:
Subtlety in the design of the of the launcher popups - Unity 3D lives up to its name with a darker shadow "3D" effect whereas Unity 2D is lighter and has a "2D" flat effect.
Unity 3D
Unity 2D 
If you don't trust your eyes you can rely on the system to tell you, just look at the desktop session variable, open a terminal and type:
If it echoes back
ubuntu-2d
that means you are using Unity 2D andubuntu
for Unity3D.But if you want a bit more details (and that's a big if), here is an extended review:
Unity 3D
Unity 3D has several features that set it apart, that are the result of it's "3D" nature and the capabilities afforded to it that way:
Unity 2D:
There are several differences in Unity 2D from Unity 3D. One of them is that the launcher items do not ever fold.
Also note the following:
Also of note:
top
orgnome-system-monitor
, you will notice that Unity 2D also consists of separate processes for the Panel, Launcher, and Dash.