The site they have used the last couple years is:
http://store.station.com/eqbeta/index.jsp
Of course, no guarantee that they will use that next time, but thats the best guess until we see the ingame spam message with the url for this time.
When open sign ups are not available, that site says (as it does now) "At this time, an account key is required to enter Everquest Beta." I think the beta is supposed to start the middle of next week, so watch for ingame spam/message board announcements.