If the examiner isn't licensed (for some reason) it could render the flight illegal in certain circumstances, so it makes sense to ask.
eg - if doing an initial PPL skills test, the person being tested isn't allowed to carry passengers. If all of the examiner's licences have expired he is then technically a passenger so theoretically the person taking the test is flying illegally. Therefore its not just him that could get into trouble (even though I seriously doubt you would be prosecuted).
Maybe we all need to be more pro-active in this? Mind you - I haven't ever asked for my instructor's licence...