All zones below level 85 are now already cross realm (i.e. multiple servers are clustered for those zones), only pandaria is now realm specific (until the next patch). Currently the only realm where I am aware that world pvp happens every day is outland, which still has a reasonably well populated horde side.
You could take a look at
this list. and look at the population column. It shows the amount of players that have killed a raid boss on that realm, and the faction relations. In theory if there would exist a realm that is full, but where only 1 player has killed a raid boss, that server would be dead last in that list. The reality however is that the amount of people who kill raidbosses is a good indication of the activity of a realm/faction.
Either way, that list should give you a brief idea.