Help with the "New" motor
 

Recently I had a used engine put into a 99 Tahoe. Having had a reliable mechanic for the last 25 years, I took the new mechanic at his word that he replaced my motor. I gave it a cursory glance-over. The reality is that I am not a mechanic. It dawned on me that I wouldn't know how to tell if the motor was replaced. Can anybody tell me what to look for?