How will i tell if the dealer really replaced my engine or not?

VCuomo

Sorry dude, none of this adds up. You clearly do not have a warranty claim, and you've already discovered that the dealer lied to you about the gas tank replacement.

But if you want to see if the engine got replaced, check the VIN stamped on the engine - it should no longer match your VIN (unless the dealer stamped it themselves, which they normally would not do on a crate engine).