XPLM_API void XPLMGetVersions( int * outXPlaneVersion, int * outXPLMVersion, XPLMHostApplicationID * outHostID);
This routine returns the revision of both X-Plane and the XPLM DLL. All versions are three-digit decimal numbers (e.g. 606 for version 6.06 of X-Plane); the current revision of the XPLM is 200 (2.00). This routine also returns the host ID of the app running us.
The most common use of this routine is to special-case around X-Plane version-specific behavior.