How to check whether an iPhone charger is original or fake?

Take the charger to an Apple Store of authorized Apple service shop and ask them to take a look and test the output.

If you bought it anywhere other than Apple or an Apple Reseller and it was very inexpensive to buy then the probability of it being counterfeit is close to 100%

Then, unfortunately, there is no way to verify that it is a real or fake product.

Although Apple does provide a website to valid serial numbers, they don't for their power adapters. Besides a serial number can be faked as well.