How could you tell if a moon or planet did not form with the rest of the solar system bodies? How might it have gotten there then?

Answer 1

The composition provides a clue. So soes the eccentricity of the orbit, retrograde motion, and its inclination to the ecliptic plane of most of the other planet orbits. Pluto's composition is a lot different from the outer gaseous giant planets, indicating that it may have been captured after the solar system formed, from the Oort cloud. Some satellites could be captured asteroids.