How To Know If Player Stopped Moving?

Checking if someone let go of the "W" key (and that only) wouldn't work, because of A, S, Spacebar, and D.

I'd check the player's MoveDirection.

Here's an example:

script.Parent.Humanoid.Changed:connect(function()
    wait(2.5)
    if script.Parent.Humanoid.TargetPoint.X == 0 and script.Parent.Humanoid.TargetPoint.Y == 0 and script.Parent.Humanoid.TargetPoint.Z == 0 then
    print("player is still.")
    end
    end)

Hope I helped!