Getting the current interface number
From WoWWiki
When loading the interface, the WoW client skips over any addons whose interface version does not match its own. This is an effort to keep the client's behavior free of errors due to changes in the API.
Each addon specifies (in its .toc file) the interface version with which it complies. So an addon's .toc file most likely contains a line similar to:
## Interface: 20403
which would mean the version 2.4.3.
Finding the client UI version locally
The WoW client's current version number is stored in (and determined by) Blizzard's FrameXML.toc file. The following assumes you've already extracted (using Blizzard's Extraction Tool) the client UI files from an MPQ archive into a directory filled with .xml files, .lua files, and one .toc file.
The extracted files end up in a directory called Blizzard Interface Data (enUS) or something similar, under the root directory of your WoW installation.
The second line of the
<path to WoW installation>\Blizzard Interface Data <your locale>\FrameXML\FrameXML.toc
file determines the current version. Note that if your client has been patched since the last time you ran the extractor, you need to extract again. The patch downloader will only update the .mpq
An online alternative
This information is also available online: FrameXML.toc courtesy http://wdn.wowinterface.com
