When we last arrived into bangkok I was asked for my boarding pass from arriving flight (first time)and proof of departure ticket out of Thailand( also first time).I had no overstays in my travels to Thailand as a sidebar.
One trick is to go up to departures level and grab a taxi there. Make sure they use their meter. But the drawback on that is the taxi's at arrivals have been vetted and the ones at departures have not been.