To access the actual heater core, if your truck is equipped with factory A/C, remove the glove compartment door and liner and you will see the black plastic molded piece which is the backside of the heater housing. Simply remove the assorted screws and the cover will come off, revealing the core.

You will then be able to see and touch one side of the core, but to get to the opposite side, you would have to disconnect the two heater hoses and actually remove the core. Honestly, if it was mine and I did not know how long it had been since being replaced, I would consider putting in a new one [~$35-50], since the labor is already done. Just a thought!