How do to make your script indicate a humanoid is falling?

An easy solution would to get the Y velocity of HumanoidRootPart and check if it's velocity is smaller than 0.

Local Script

local player = game.Players.LocalPlayer
local character = game.Players.LocalPlayer.Character or game.Players.LocalPlayer.CharacterAdded:wait()
local humanoidRoot = character:WaitForChild("HumanoidRootPart")

while wait() do
    if(humanoidRoot.Velocity.Y < -0.2) then
        print("Falling")
    end
end

The reason it's not just 0, it's just makes it less likely to detect a step as falling, you can set it to 0 if you don't like this.