It’s quite easy to spot a taxi in Valencia; they are white and sport the word taxi on top. Spot a green light on the roof and you know they’re available. Just wave your hand to flag them down.
You can keep an eye on the fare by looking at the meter. Taxis are relatively cheap here, but there is a minimum charge, 3.90€ during the day and 6.00€ at night. Occasionally there are some extra costs as well, at the airport or the port for example. You can see these charges written on a window sticker.
Best to make sure you have change of 20€ or smaller before getting into the taxi. Drivers aren’t obligated to have change for larger notes.