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:

echo $DESKTOP_SESSION

If it echoes back ubuntu-2d that means you are using Unity 2D and ubuntu 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:

• The launcher items fold when there are too many windows open or excess items pinned to the launcher.


• Also, the trash can icon in Unity 3D is based on your current icon theme, and thus looks transparent in the default setup (when empty).


• The Panel and Dash change colour to match your wallpaper when the Dash is open, and the launcher is mildly translucent by default.


• The Dash icon is black on white, with a transparent border, and rounded edges.


• Unity 3D is a Compiz plugin and requires Compiz, which means your graphics card must have 3D support, and you cannot run it on another window manager; so if you are running anything but Compiz with Unity - you are definitely not running Unity 3D.


• Unity 3D's icons have a nice gloss+glow to them, and transparent/translucent edges.


• Unity 3D takes advantage of compositing fully even for quicklists and Launcher item tooltips.


• Unity 3D's panel has a shadow.


• Unity 3D has a fully 3D workspace switcher with smooth animations and an orange glow.

PLEASE NOTE:

These images present a slightly modified launcher, in that I have the Launcher set to "Edge Illumination Toggles" instead of the default "Backlight Always On".

• Unity 3D's Launcher with folded icons.

• Unity 3D's Trash Icon

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:

Notable differences in Unity 2D

1. The Dash button is not translucent.


2. Other buttons are flat icons, and not transparent/translucent with a glossy appearance as in Unity 2D.

Also of note:

• The launcher and Dash are not transparent when not using a composting window manager such as Compiz or Mutter (for example). In the Default install, Metacity has compositing enabled.


• Unity 2D can run in different Window Managers, unlike Unity 3D. If you run top or gnome-system-monitor, you will notice that Unity 2D also consists of separate processes for the Panel, Launcher, and Dash.


• As noted by fossfreedom, Unity 2D does not currently take advantage of compositing for Launcher item tooltips and quicklists.


• The Panel + Dash do not adapt to your wallpaper when the Dash is open.


• The Panel has no shadow.


• Unity 2D's workspace switcher has no glow, and has slow animations with no smoothness to them.


• When there is an overflow of items on the Launcher, there is no folding. The launcher only scrolls.