Note the phone number written on the taxi door. And also note the ID number of the taxi (inside, most of the time).
If he refuses to turn on the meter (you can say ‘tick tick tick and point towards the meter), tell him you will call with a complaint, and call.
2 times I tried it, the taxi driver instantly agreed to turn on the meter. Apparently it is illegal for them to refuse in Bangkok.
The airport may be different, especially ‘leaving’ the airport.