[O] Search for defined property

[Pete Ley]

I have trees in several agenda files with Effort properties and I'm
trying to make an agenda view that shows all trees with this property
defined. 

I know I can do 'M-x org-agenda m' to do a property search but this only
seems to work searching for properties with a specific value. I'd like
to see a list of all my trees I've committed Effort to. In my main org
file I have a clock report which includes the Effort property and this
works well for seeing Effort vs time worked, but for things I haven't
clocked in to yet obviously nothing shows up. 

Is there a way I can view all trees which have a certain property
defined or a (fairly simple) way to restructure the way I do things to
accomodate this? 

Thanks