How do make a script detect when [player] is sitting?

You can use the humanoid's Seated property

Put the following code in a LocalScript inside Explorer Tab->StarterPlayer->StarterPlayerScripts.

local character = script.Parent
local humanoid = character:WaitForChild("Humanoid")
 
--this is an event that is called every time the player's Seated property changes
humanoid.Seated:Connect(function(isSeated, seat)
	if isSeated then
		if seat then
			print ("The player is now sitting in the seat named "..seat.Name.."!")
		end
	else
		print("The player is no longer sitting!")
	end
end)