Like Jay said, they stopped making the Zetec in the Focus in '05. From '05 on, all Foci have Duratecs. You can find some Duratecs in the '03 and '04 models, but most of them are from the New England or Cali areas.
The easiest way to tell the difference is visually. Zetecs will have a Zetec logo on the valve cover, but if there isn't one visible there are two easy ways to tell the difference.
1) The Zetec engines have a distributor cap with spark plug wires; Duratecs have coil-on-plug wiring.
2) The Zetec engines have the intake on the back side of the block towards the firewall and the exhaust on the front side behind the radiator. The Duratecs are the reverse - intake in front, exhaust in back.