First of all: Unless you use Uber or GrabCar, which are basically banned in many less-central areas, always ask the taxi driver to put on a meter. If you choose BlueBird taxi, the safest of the legal taxis, they tend to use the meter automatically, except during night time. Second: If you live somewhere a bit further away, try not to get into a taxi alone, at least not during dark hours. There are always drivers masquerading their cars to look like the common companies, and harassing you on the way, or even worse, robbing you and leaving you stranded in the middle of nowhere.
The scam I wanted to mention about, is the sneaky change of the bill you pay with. Some dishonest taxi drivers know it is sometimes difficult for tourists to notice the difference between a 100 000 bill and a 10 000 bill. When receiving your 100 000, the taxi driver switches it to a tenner without you noticing, and then shows it to you and insists you haven’t paid enough yet. Unless you want to end up paying five times the going fare for a short ride, please hold the bill in your hand until the driver starts digging his pockets for change.