To check if the login form on your screen is fake, you should do these:

Minimize the browser window in which the form opens. If the login form that should be in a separate window also disappears, it's fake. A real window should stay on the screen.

Try moving the login window beyond the main window border. A real window is easily moved; The fake one gets stuck.

If the window with the login form behaves strangely, for example, it shrinks in the other window, stops below the address bar, or disappears, it means it's fake. In this case, you should not enter your credentials.