if you watch mary and oaks radio show, they would say that there is 'breaking news' and say that a Pokemon (kind depends on the swarm) and there location. this is how you know what what piokemon is in a swarm on that day. this happens daily.
this radio show also tells you the location of other Pokemon throughout both regions.
in diamond pearl & platinum ( I might as well mention) you can tell if there is a swarm by a: watching tv until breaking news about swarms comes up, or b: asking lucas/dawns little sister in sangem town and she will tell you.
hope I helped (im pretty sure this would)