Good advice from cat. If there's any chance if handing your note in to the doctor earlier then that's a good idea too. In that way he/she will have read it all before your mam arrives. I'd also take some notes if what you're told. I always found it hard to concentrate on everything that was being said.
I also found it good to sit slightly behind mum so that the consultant can see me but as i'm not level with mum I can signal or 'mouth' words to indicate what mum is saying is not accurate. In the early days mum was so convincing and it was if she knew she needed to put on a performance when she was with doctors and other professionals. A friend who had walked the dementia path gave me the advice of sitting slightly behind mum so I would be beside her but just out of her line of sight. It really did work. I also agree with Cat and Izzy that writing down a list of your concerns and passing it the person booking in your mum at the appointment is a big help as the doctor then has a better idea of where to start. They are all so used to this. I used to book mum in and then say "oh I just need to check something" and then go back to reception and hand the note in.
Please don't view this first appointment with dread rather view it as a positive step that will mean your mother gets the help she needs. Alot of help is offered on the basis that someone is known to memory services so this is a good step for you and your mum.
The doc kept looking at me for signs I think I blinked or scratched my nose or touched my head if I knew mum was lying or not telling the truth. When they took mum away for her tests the Doc then spoke to me re the notes she made.