In Dwarf Fortresss, how do you verify that the civilization you're embarking from isn't too small to provide migrants?

The wiki states: "There is just ONE BIG RULE: when your home civilization is too small, you will recognize after the second winter that you won't get any more immigrants, which can be extremely fun. To avoid this situation, select a home civilization with at least two dwarven sites on the map."

How do you determine that a civilization has at least two sites?

Solution 1:

Before you embark, take note of your civilization's name. Look them up in Legends mode. It's easiest if you have Legends Viewer installed but it can be done in the base game as well. You should be able to see a list of all the sites that civilization controls.

Just a note, you don't necessarily need more than one other site, so long as that site is large and safe.