The problem most seen with unintensional dual boots is when someone runs the installer from within an existing copy of Windows and selects the custom option and ends up pointing to another partition or drive and not simply upgrading over the present. The fresh install is still made the default OS automatically.
In that case a look in Windows Explorer for all logical drives shown will generally indicate another Windows installation is present since Windows will automatically assign a drive letter usually D for the other. That offsets the default D for optical drive on the fresh install.
It may also see the E, F, or G drive letter assigned to it provided the previous volume was mounted during the fresh install of 7. Then you would need to go into the DM to locate the volume and right click to assign a drive letter to see it initialized. But that all brings us right back to the need for a look at what the DM will reveal.