how to compare an object to nothing when the operator '=' is not defined for that object

Hey, I am writing an application for one of my classes and I was wondering if anyone can help me with this problem.  I want to see if an object passed back from a function is equal to nothing.  When I just type obj = nothing, i get the error that the operator '=' is not defined for this class.  The class I am trying to check is a class I wrote.  Is there something I can add to the class I wrote to allow me to use this operator or some way I can compare it to nothing.

Thanks,
Kevin