Who would give diagnosis for Alzheimer's?

A neurologist or geriatrician are qualified to diagnos Alzeizmer's. M.D.s do but really not qualified. Alzeimer's can be misdiagnosed and many times is.

Can't say how the character would know. Alzeimer's patients are very sneaky and if they don't want someone to know then they just won't know. Only way is if a family member knows and spills the beans. One has to talk directly to the Dr. to find out.

If there's a way to do it you might want to throw in something about memory testing. My Mother-in-law was recently dx's with dementia - which is probably used more often than "alzheimers" as a dx. She had been referred to a PhD psychologist for testing and diagnosis by her neurologist. Neurologist first dx'd dementia but wanted more info to assess whether it was alzeiheimers or not. She did not know what the testing was about, only that it was very frustrating and she kept feeling she was failing and disappointing the woman who did the actual tests. The psychologist meet with the family to explain the diagnosis (dementia) and to discuss things to do/not do. As I think about it I doubt if many professionals assess patients for a dementia without family, staff, etc. to discuss the furthering needs of the patient.

Our local GP originally diagnosed my aunt - but she was then sent to the Memory Clinic, where she was reassessed by a psychiatrist. (She had to do those tests too.)

My stepdad was diagnosed recently. It seems they have some sort of blood test for it now.

I would think a neuropsychologist or gerintologist.