If it was possible to have a preview of what a camera will see when you select it in the editor, as either an overlay in the main 3D scene or as an image in the Properties Panel, that would make positioning of it in the scene a lot easier.
A sort of workaround is to launch the game in a separate window and move it in the editor and view the change in the game window, but this isn’t that intuitive for new users, requires more steps than just having in-editor functionality, and if the game model to track is moving when you run the game, it would be more difficult to get the initial position set in this way.
So open an Editor in one window, and Launch in another. If you have 2 monitors - it is best. If only one monitor, you can put editor on left, and launcher on right side of a screen. I recommend double-monitor workflow.
Then select in viewport camera as @will has shown, and then fly that camera. If your code in launcher not overriding position and rotation of your camera, you will see that it will move, so you have a preview and it allows to move and rotate camera in real-time.
Just checking, as the solution you offer here sounds very similar to what I had put as a workaround in option in the first post ( second paragraph ).
I was suggesting that a smaller second window be visible in the editor when you select an entity of type camera, so that when the user selects a camera and starts using the gizmo to move the camera in the editor ( translate, rotate, change FOV etc ), they will get a pretty good idea of what you will see in-game without having to do anything else ( open new windows, set up a second monitor etc ), it’ll just work.
Other editors, such as Unity and Flare3D IDE, do this, and it is very intuitive and easy for new users ( and existing users ) to use.