If the iDevice Manager is not yet installed, you can download the program here.
Connect your iPhone to the PC with the USB cable.
Start the iDevice Manager and click in the left menu "File Explorer".
The folders from the iPhone are displayed in the tree structure on the left.
In the right half can you see the files of the selected folder.
By clicking on a folder, any existing sub-folders are displayed as an arrow symbol.
You know this view from Windows Explorer.