how to get a list of all nodes that haven been created and are not in use on the system as the title says.. 
i need to get a all published nodes of type A that have been created and not used (at the moment) by other nodes.

in other words a list of all node of type A that are on the Database except for those that are NOT been referenced in an entity reference field.

the easy approach will be to load all nodes that uses node of type A, and that load all type A nodes and check for each if it's has a reference for it.
but then if i add another node that will use node of type A as an entity reference i'll have to go back check it as well.

is there an easy way to do it ?
You could set up a view showing title of content type A,  title of content type B (using relations), hide both and create a custom PHP field that just returns title of ct A if ct A does not have ct B referencing to it. Maybe