how do I check If a player is moving?

local Player = game.Players.LocalPlayer
local Character = Player.Character or Player.Character:Wait()
local HumanoidRootPart = Character:WaitForChild("HumanoidRootPart")

local Velocity = HumanoidRootPart.Velocity.Magnitude

while wait(0.01) do
if Player.Character.HumanoidRootPart.Velocity.Magnitude >= 16 then
		-- Do stuff here
		end
end

Create a LocalScript in StarterCharacterScripts and insert this there.

Hope this helps!