How do you find out if it is in a swarm?

Question

Im trying to get some Pokemon that is usually found in a swarm? How often does this happen? Will I be notified when it happens?

Answer 1

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)
yours truly,
loomhigh123555

Answer 2

Info about the swarms can be found in Sandgem Town by talking to Lucas/Dawn's sister, by listening to Professor Oak's Pokémon Talk(HG/SS) on the PokéGear Radio, or by watching the Sinnoh TV (D/P/PT).

If you're playing Gen 5+, apparently the gates in Unova have a monitor inside that alerts the player when a swarm happens.

Source: http://bulbapedia.bulbagarden.net/wiki/Pokémon_outbreak