To find where it is before seeing it the first time?
If you look on your town map or your Fly map it will show a [email protected] head. But if you fly to a place, it will just move to a different location. I suggest you walk to it's location.
Look on your map and there could be a head of Latias or Latios there. Sometimes it won't be there. What I do to look for it is fly to a city then it will suddenly appear on a different part of the region.