How can a mechanic claim I have original plugs/wiring harness?

Sometimes there are tell tale signs that parts have never been replaced, but sometimes people can make mistakes. I would recommend consulting your dealer to obtain full maintenance records - if possible. And/or check your own records to see what the history of work has been. This will make you an educated owner, if nothing else.

theres a date on your wires possibly

SOMETIMES OE PARTS ARE MARKED WITH A SLASH OF COLORED PAINT