Click "Help" and select "About Eclipse SDK"
This will bring up this popup: 
In it, click the button with the Android logo in (highlighted in red), which will be displayed:

This will show you the current version of ADT.
Alternatively, you can also go to the Help menu â Install New Software. Then select the ADT plugins in the "Work with" section, expand the "Developer Tools" option and see the version currently installed.

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