“I can’t remember.”
“I do not recall that.”
“I’m sorry but I have no recollection of that.”
Confident liars will say anything, but when they’re under oath, they often say something like the quotes above. Prosecutors know this, and they also know why the liars do it. It’s next to impossible to prove that someone does remember something he or she claims not to remember. I suppose the only way you could do it would be to trap the person with a separate line of questioning in the hope you can get the person to answer something else without realizing they couldn’t possibly know the answer to the one question without also remembering the thing they had just claimed to have forgotten.