How to Determine if 54 3100 has original engine?

[DENALI]

This truck was bought new by my grandfather and has been in the family its entire life. I know that at one point it received a new or rebuilt engine (35+/- years ago) and parked a few years later. Its long been locked up and I have found a possible replacement engine that has been newly rebuilt to get it going for now. How do i determine if the current engine is the original so I can keep it stored for the truck if that is the case. THanks