Short together the 2 pins of the service check connector (a cut-off U bend from a paper clip works well). The connector should be in the passenger side footwell near the blower motor... It's blue IIRC.
If there are no codes stored, the check engine light will come on and stay on solid as long as the connector is shorted. If there are codes stored, the check engine light will start flashing them out in a repeating series. Short flashes stand for 1, long flashes stand for 10. If there are multiple trouble codes, they will be displayed in order with a short pause between them: 1st code, pause, 2nd code, pause, 3rd code, pause, 1st, pause, 2nd and so on.