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: 20400

Here, 20400 would mean that the addon is designed against the Patch 2.4 client (and any subsequent minor releases).

There are a number of ways to get the current interface version:

It probably is 30200 

But this number is maintained locally, so it might be out of date.

Steal it from another addon 

Recently updated addons, which are not listed as "out of date" by the client contain the latest Interface version in their toc tag.

Use GetBuildInfo 

In particular, /run print((select(4, GetBuildInfo()))); should output the correct version to your chat frame.

Extract FrameXML and check FrameXML.toc

You can download Blizzard's User Interface Customization tool, use it to extract interface data, and peek at World of Warcraft\Blizzard Interface Data (usUS)\FrameXML\FrameXML.toc

View FrameXML.toc online

For instance at wowcompares.