Create a View with a filter.
The condition for the filter would be:

1. Select field "Checked-out to"
2. "Is not equal to"
3. Leave value empty

Create a View with a filter.
The condition for the filter would be:

1. Select field "Checked-out to"
2. "Is not equal to"
3. Leave value empty

And to see all files regardless of folders specifiy the Folders setting:

Folders or flat: Show all items without folders