Mazda Repair: How to make sure if my mechanic truly has replaced the alternator, core deposit, mechanic

Question
Few days ago my car's electric system failed when I was on
the highway, and on the exit, engine shut down and didn't
start again. Car was towed to a workshop and the mechanic
told me that alternator needed to be replaced. When the
alternator was replaced and I came back to pickup the car, I
asked the mechanic to give me the old alternator. On this he
made excuses that he couldn't return the old alternator
because it needs to go back to the manufacturer. I asked him
to show the parts replaced and on this he made some other
story why he couldn't show me the parts. This has made me suspicious on whether he actually put a new alternator in at
all, or just replaced a broken belt, or tightened some lose
wires.

My question is, how can I make sure that the alternator was
actually replaced and I was not ripped off.

Answer
you can tell normally just by looking at them it'll look clean and new as for not being able to show you the old one that would be true alternators have a core deposit and you have to return the old one they rebuild them alternators are hardly ever new now a days there rebuilds however there sould be a warrienty so if it does goes again the part is replaced free of charge however the labor will not be covered