How can I check if my new iPhone 16 Pro Max is already activated?

Please check the information in the link below, especially the Pre-Purchase Check List. Essentially, your new iPhone should open to the Hello screen which then allows you to set it up using your Apple Account information. If it does not, return it for a refund.

The All Too Common SAD Reality of Buying … - Apple Community

So you bought a used phone… not a new one.

If you don’t trust the seller, return it and get your money back.