I doubt there's any way of detecting this. The GFX card has output only, and that's it... unless I'm wrong
There is some very basic two-way communication going on as well -- pretty much all monitors nowadays are plug & play, and can supply the computer with a list of supported resolutions. XP will also recognize the make & model of most screens.
So... You may be able to just retrieve the name of the installed monitor from the registry somewhere, and go from there?
(Even if not explicitely stated, you may be able to figure some things out based on supported resolutions & refreshrates as well)