All the objects that I am using are part of my forest library. Here is an example:
I have a blank scene. I make a forest object, and a surface to place trees (for instance) on. The forest object is set to "custom edit". I go into the library and select 5 different trees and add them to my forest object. I then edit my forest object and place 10 trees, but specify only 1 of the models that are part of my forest object, as I have decided that the other 4 are not correct. I therefore have 4 models referenced into the scene that don't need to be there.
In this particular case, I can very easily work out which trees are not in use, say by colour, or by just interrogating the forest object to find out what models are used.
Now imagine that there are 15 forest objects that have each been edited and added to by 3 different people in the office, and the file is now VERY heavy. There are 66 objects on the "Forest templates" layer, but I am almost certain that some of them are not actually used in the forest objects, as described above.
Does that help? How do I see if any of the models specified in a particular forest object is being used or not?