We have bought a 35W charger. How can we verify its authenticity?

If you are purchased the charger from an Apple Store or from Apple online, then you know that you bought a genuine Apple product.

Otherwise,you do not know if the charger was genuine. However, if the charger is working well, is it really that important that it might not have been from Apple?

If you buy it from Apple or an Apple Authorized Reseller, then you will get the genuine product. Everything else is uncertain to a degree.

You don’t strictly need an Apple charger (power adapter) for your Apple devices, though. Any good charger will suffice, up to the rated Watts as maximum possible energy throughput.