A 90 is OBD0 unless you have converted it to OBD1.
If it's still OBD0 there should be a light on the ECU. Turn the key to the "ON" position, then watch the ECU for the flashes.
If you've converted it to OBD1 jump the connector under the dash with a wire or paper clip. Turn the key to the "ON" position, and count the flashes of the Check Engine Light.
Also, you could have searched and found this quite quickly.