In the screen with the three maps, when in the embark location picking processs, press TAB. This will take you to a new panel, that shows other info. One is the Civilization panel, and with up/down arrow, you can highlight other dwarf Civilizations. The one you leave highlighted, right before smashing down TAB again, will be the Mountainhome, where your 7 dwarfs originate from.
And it will, on base ASCII also show on the larger, right most map, the mountain halls will highlight a different color than white on that map, inplace of the many mountains, or Δ. depending on how large that Civilization is there could be more mountain halls. This is dependant on how it did during the History roll out up to the embark date.
The more Civs ya permit in the Worldgen parameters, the more of these ya have to choose from. Defaults have very few. If you do decide to increase the number of Civs, be sure to increase sites and lower populations per site, or you'll have nowhere to embark that isn't already snatched up.
Each mountainhome has its own relationships with the other entities(race/site/civs), and its own stuff. Like some worship megabeasts, and some don't. Some are at war, or in the middle of a 12 year campaign. They can be isolated and read about prior to making the choice, by using Legends Viewer.
I used to do that, but now I just pick the one farthest away, so I don't get pestered by awful large swarms of insanely overskilled migrants, each year. Not sure if this actually does anything, but its how I play. I don't like to be close to authority, or the higher ranking nobles.