Getting the current interface number

When loading the interface, the WoW client skips over any AddOn 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:
 * 1) Interface:

The version string is (most of the time), the major patch number but with any periods and leading zeroes removed. For instance, 90002 is 9.0.2.

There are a number of ways to get the current interface version:
 * It probably is (Retail) or  (Classic): But these numbers are maintained locally, so they might be out of date.
 * Use : In particular,   should output the correct version to your chat frame.
 * Steal it from another AddOn : Recently updated AddOns, which are not listed as "out of date" by the client contain the correct Interface version in their toc tag.