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)