How to know when an animation loaded into a character stopped?

Once you've loaded the animation and have begun playing it, you can stop the animation with animTrack:Stop(), where animTrack is the variable where you load the animation into the humanoid.

You can use the AnimationTrack's IsPlaying (a bool value) to tell if the animation is playing or not.

There is also an event .Stopped

animTrack.Stopped:Connect(function()
	print("Stuff")
end)