"I've heard it said you should never ask a question in cross you don't know the answer to. I guess that's not always feasible, eh?"
No, it's not always feasible to know the answer to a question before asking it. However, I find that most of the time police officers are willing to talk before trial and between what the officer says and what your client says you can usually have a pretty good idea what questions are the ones you should ask.
Important Practice Point: Do Not ask that Perry Mason moment question of your client's ex-girlfriend. She will lie. She is emotionally committed to getting your client thrown in jail. She is also angry with your client because he dumped her for that *%^&^%. She will lie. I know the question is tempting to ask because if she gives the honest answer your client should be cleared. Leave the doubt in the air, don't give her a chance to tell some tale. A failure to prove can be potent (if used properly); a lie from the woman on the stand can be devastating.