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Internal is driven from center. External from whatever body part is moving.
The origin of the terms is pretty much self-evident.
If you feel a directional force on the point of contact, it's probably external. You can test this by pulling away suddenly. If your partner follows you, leaving themselves open, it's definitely external.
If there's no force you can identify on the point of contact and you're falling anyway, and you didn't throw yourself off balance with a dopy attack, it's internal.