1998. 5 and older the codes flash with the check engine light in a sequence... the new odometer method is easier i think tho. .
to do it on either, turn the ignition on and off a total of three times, on the third time, leave the key in the on position (do not start) and watch the odo. You can also do a gauge check by holding in the odo reset button and turning on the truck, then releasing when it starts to do stuff. .
-j