Did she have long-time friends that no longer come around or contact her? Find some pictures of her (preferably in a group) from present to about 10 years back - does she look lively, life in her eyes? Or is she looking blankly in the direction of the camera and maybe not smiling?

When I went back looking at pictures I was surprised that I never noticed that on pictures when we would go on vacation with other people she was always kind of disconnected. Well after the "idiot" stuff started she would leave to go to the store and be gone way longer than one should be. I realized later that she was actually getting lost.

Just observe the behavior - also if there is Alzheimer's in the family be even more observant.