Off the top of my head. You're right to say it's not that simple. My guess is your mum might be difficult to diagnose. She may be asked about sense of smell, balance, tremors, handwriting, falling, pains, constipation and more. And if my mum in law is anything to go by, difficulties in any of those areas may not (or may) indicate PD. i guess you're used to accompanying your mum on hospital visits? So you'll know to be prepared to check your mum's being realistic in her answers, and at the same time to take on board what the dr says.
The question I might ask would be - if not PD then what? If you can't dx now then when? Whether you can or can't dx, what can you do to alleviate symptoms?
And if you want some light reading before be, look through the booklets on this site: I'm sure there used to be one valled something like "my first neuro appointment"
Best wishes to you both