I generally ask them to describe the process architecture of WebSphere -- if they don't know (for instance) how many admin servers there are per node, or how cloning works then I don't continue with them. If they know this, then they've probably used WebSphere enough to know their way around.
After that, I'll generally ask standard
J2EE questions -- explain how Session Facades work and why you would use them, explain why you would or would not choose to use EJB's in a particular project, explain what the components of an MVC architecture are, etc....
Kyle