How to know if my new adapter is Apple certified or not?

One method is to look closely at the labels printed on the power adapter. Compare them with those in the article I provided you. The lettering should be "clean" and the label itself should not be a-skewed.

The other tell-tale sign is the price. Did you get this adapter at a very low price? Knock-offs tend to available very cheaply.

Another is the return policy. Can you return it without any cost ... or even will they take returns? Apple would provide you with 14 days from the date of purchase to either return it for a full refund or a replacement.

Did you purchase directly from Apple or an Apple Authorized Reseller, then it would be authentic. If not, then there is a possibility it is a fake.

Ref: About Apple USB power adapters - Apple Support

The article I mentioned is in the link that I provided you earlier. Here it is again: About Apple USB power adapters - Apple Support