How to determine which theme is currently enabled

You can use gsettings to show currently enabled gtk theme:

gsettings get org.gnome.desktop.interface gtk-theme

and set eg. Radiance

gsettings set org.gnome.desktop.interface gtk-theme Radiance

If only want to know which themes are available on your system you can look into these two directory:

• /usr/share/themes
• ~/.themes

The above directories contains the themes folder.

If you also want to know which is the current active theme you can follow these two way:

• Open System Settings and then choose Appereance. Now in the Look tab watch at the window bottom right: you can see which theme is currently set (e.g Ambiance);

• Install unity-tweak-tool to manage unity appearance:

  sudo apt-get install unity-tweak-tool
  

  Now open it and select Themes section. It will be shown the list of themes installed on your system and you can choose which is the best for you or you can install other from internet.

Note: if you have installed a new theme, then you can't see with the first method. Unfortunately you need a third-party software to manage themes different from default.

EDIT:
I you want to know which is the current active theme from terminal you can use this command:

gsettings get org.gnome.desktop.interface gtk-theme

I hope this could help you.

18.04 and later

In Ubuntu 18.04 and later open the Tweaks application from the Dash and click Appearance in the left pane. The currently enabled theme is shown under the Themes heading and is marked by the mouse cursor in the below screenshot.