I wonder if it's possible for a tourist to actually get to his destination without paying extra you know just paying what the meter states today.
If you are a tourist to thailand you would know that almost 90% of the time if you get a taxi from the airport they will charge you a certain fare usually higher than what the meter will give.
I don't think I recall ever being egregiously overcharged from the airport.
If you take the taxi from the official taxi rank, they have to use the meter. If they don't then get out. If the meter is already running before you start journey, get out.
I don't take a taxi from the taxi rank but instead, from the departures floor - I save 50B - but I kind of think you'd almost have to a) be a frequent visitor to Bangkok and b) know some rudimentary Thai.
Of course, you always have the risk of a rigged meter but nothing much you can do about that. If you look at the bottom right (I think), it will show the distance in km so you know roughly if it is running hot. The casual tourist also won't know how much roughly will a fare be or even rough distances so this info might not help much.
Finally maybe use GrabTaxi? I've used this a couple of times and although you pay a little more (20B call fee or something like that), the drivers have been very honest in my experience.
I've been lucky perhaps but good luck to all.