How can we tell when my father doesn't remember something or is just not listening?

Pamstegma is correct. Look Echolalia up for yourself.

Well as an old fart myself, I have selective hearing, I hear what I want to hear. Dementia is not funny, but there are ways to see if he remember, if you are going to the doctor's office and he ask, where are we going? Tell him you are going to the grocery store. If he knows where he is going he'll reply I thought we were going to the doctor. Then you reply; no I am going to the grocery store; you are going to the doctor. Play the same game he's playing. If he truly has the beginning of dementia he will not reply. Just don't say anything if he gets nasty.

My mom often falls back on the "just joking line" when called on some outrageous comment. I think she can tell - even as out of it as she is - by our faces when shes crossed a line and thats her fall back excuse. Not funny tho, example doc asked her how she got some new bruises (falls) she points to my brother who took her to appt and says he did it. Doc says "what???" she says "I was just playing". Like I said NOT FUNNY.

If he is playing a game... just tell him You;ll see when we get there, it;s a surprise.. and see if he suddenly "remembers".. And if he is being nasty. just ingnor it.. tell him you;ll speak to him when he remembers his manners. However, if he has dementia.. this many only make him angrier. But my dad hated to be ignored...He would get much nicer if he was not getting attention.

He may be hearing you, but his brain does not process the information, so he repeats it back. It is called "echolalia"