If the meter isn't on when you get in the cab, ask for it (tip: "meter" in Thai is... drum roll... "meter"!)... and if you have lots of luggage, ask the cabbie before you load it in. If they refuse to turn it on, get out and get a different taxi.

If waving down taxis I ask if they can go to my destination with bpai X dai mai na khap, after which any cabbie will reach the (correct) conclusion that this guy has been around the block a few times, and will either turn on the meter with no complaints or drive off in search of another victim.