How do you tell if you have the correct engine in your car?

How do you tell if you have the correct engine in your car?
by correct do you mean the engine that was installed by the factory? if yes,there are several small idenification plates affixed throughout the car , mostely on major sub assyemblies ie.: transmission,engine,body,rear axel ect,ect. all the numbers will match if the part is original when manufactured.