how do I know our own product is genuine or counterfeit

to make sure you have genuine product, you can check serial number or imei number of product with apple here is link of Service and support https://checkcoverage.apple.com/nz/en/

What product? Apple makes many different products. If we knew the product that concerns you we may be able to suggest specific things for which to look.

We cannot possibly tell if your own products are genuine or not.

We can possibly comment on Apple products if you provide the details requested.