How Do I know when a player stopped walking?

I want to make a script that a player dies after he stopped walking. But I can't find the right Property to find it? Can someone help me??

Here's how my finished script would look like:

--LOCAL SCRIPT

player = game.Players.LocalPlayer
Person = player.Character
DontStop = false

game.Workspace.Triggerpart.Touched:connect(function(hit)
if hit.Parent.Name == player.Name then
Person.Humanoid.Walkspeed = 100
DontStop = true
end

--Help here: When the player stops walking and DontStop == true then...
Person.Humanoid.Health = 0

Your help will greatly be appreciated. If you wish, your username may be posted under the Minor Credits section in my game.