How do astronomers confirm the internal findings about the Milky Way's overall shape?

Answer

By comparing internal kinematic data with the observable structures of completely visible external galaxies used as templates.

Since we cannot view the Milky Way from the outside, scientists confirm their derived model by finding external galaxies that match the inferred rotational speed, mass, and structure (like having a bar).

How do astronomers confirm the internal findings about the Milky Way's overall shape?
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