How can I verify that my backend and frontend are connected, and how do I properly connect them?

I am working on a web application where the backend and frontend are developed separately.
The initial project structure and code were generated using an AI tool (Cursor) and I am now trying to understand and verify the integration between the frontend and backend.

How can I verify that my backend and frontend are successfully connected?
Also, what are the correct steps to properly connect a frontend application to a backend server?

Specifically how can I:

Test the backend API independently

Confirm that the frontend is correctly calling the backend API

Identify common issues such as incorrect ports, CORS configuration, or environment variable problems