That is a really good question. I wish I knew. It'd be nice if the XFree86 configuration was smart enough to detect stuff like that. The same could be said for other stuff that is often a pain for newbies (and even oldbies) to figure out, like mouse configuration. Best Linux mouse configuration I've ever seen was in
muLinux; it said "please move your mouse around for a while so I can detect it." After moving it around for a few seconds, the cursor moved with it; configuration done. Why can't they all do that? You'd think it'd be easy to probe all the common ports to see if there's mouse-like input coming from them, so people don't have to know that their mouse is attached to /dev/psaux or whatever.
I guess for monitors, it's not too safe to try a bunch of video modes to see what works, since the wrong one could make the monitor explode or something. But surely there's a way... monitor refresh problems are definitely one of the most common things we see here on LQ. Linux isn't very useful if your display doesn't work right.
Eric S. Raymond wrote a
great piece on the shortfalls of Linux configuration recently. I'd urge everyone to take a read - especially if you're in a position to do Linux development.