Which tasks can be visually confirmed by other crew members?

The following tasks have visual indicators:

MedBay: Submit Scan (The Skeld, Mira HQ, Polus)

While scanning, a green hologram will surround the crewmate, moving up and then down.

Shields: Prime Shields (The Skeld)

Upon completion, the shields to the lower-right of the room will activate and turn yellow. If this task was already completed, completing it again will cause the lights to pulse.

These shields are off:

And these are on:

Storage Room: Empty Garbage (The Skeld)

This is a two-part task. The second part always takes place in the Storage Room, and upon completion, you can see garbage ejecting from the bottom of the ship.

Weapons: Clear Asteroids (The Skeld, Polus)

When the crewmate fires the weapons (by clicking / tapping on the screen), the weapons outside of the Weapons room will fire a laser. On The Skeld, these are on the right of the room ; on Polus, these are outside the room, on the left, and are visible from outside.

This is where the weapons are on The Skeld:

This is what it looks like for weapons on The Skeld to fire: *

This is what it looks like for weapons on Polus to fire:

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^{* Image credit: Among Us Wiki}

1. Medbay scan can be verified during the scan by seeing a green field around them while they are doing it.
2. Asteroids/weapons will show the cannons firing during the task
3. Some trash tasks such as bottom trash on Skeld will show trash falling out upon completion.
4. Shields on Skeld will show lights that turn on outside the ship for the first person who does it.
5. All tasks that are not multipart tasks will have the task bar move when completed, and the final task in a multipart task will also cause the bar to move. Multipart tasks are things like Wires, Upload/Download, Trash, Repair node.

This all depends on the game setting though, as you can turn off visual tasks and the task bar verification.