RT Assests. How to search for assets that have no 'Held By' valeo?

Hi Everyone,
Long time RT User with what i hope is a simple question. I have installed
the Assets extension and imported lots of data with the Assets Import CSV
extension. Some of my assets have ‘Held By’ information and some do not.
What i want to do is perform a search that will display all of the assests
that do not have ‘Held By’ information. Maybe i am missing something but
there does not appear to be an obvious way to do this ?.
Thanks in advance for any assistance.