My standard answer is...
..."I don't know, but I know someone who does" or "I don't know, but I will find out." The trick is to be confident, even when you honestly don't have the answer at the tip of your tounge. That confidence reassures the client that even if you don't have the answer on the spot, that you still have their best interests as a priority, and that you will find the correct answer one way or another.
Few people expect everyone to know everything about everything. But they'll more than likely respect your integrity in being honest, and your enthusiasm for getting the right answer.
When I started out in IT, it was nearly possible to know everything about everything. There were only so many platforms, OSs, languages, and applications to know about. And I prided myself about knowing something about all of it. But by the latter half of the '80s with the explosion of IT, it was no longer possible to even know even very much about "everything", and impossible to know enough about much of it. You had to prepare to be humble.
