My MazdaSpeed 3 engine blew up, so I bought a new one. When I saw my new engine, it was painted in gray color. My previous mechanic said it was brand new and he bought it from a company from eBay. When my new mechanic saw the engine, he said it seems like the engine came from a dismantling shop. He said mechanics have a term for these kind of engines and they call it "Mexican Chrome". What the dismantlers do is they paint their engine gray to cover up the dirt and other damages and sell it off as is. My new mechanic was not sure about it, but I am certain my previous mechanic just ripped me off. I don't really know what to do. The previous mechanic said he bought the engine for $4500.
How can I tell if the engine my old mechanic bought and installed is new?