How do you find out how far someone is into Alzheimer's disease? What does the early stages of Alzheimer's mean?

I think my mother-in-law was starting to show signs of dementia while her husband was still alive. He was covering for her failing memory, because we sure never knew the scope of her brain problems until he got sick and eventually died. She was thrown full blast into dementia from that shock I believe and has never recovered. Just recently her G.P. put her through a pretty long test of answering questions, remembering specific things etc. (which she failed miserably) and told me that she has early Alzheimer's. Honestly I never thought I'd have to hear the 'A' word. I just thought she'd stay in dementia land for a long time....wrong. For her, it's the memory thing that's taken the biggest hit. She literally can't remember something I tell her for 2 minutes, it can be very frustrating. So maybe it depends on the person as to what early alz looks like, I don't know. She still has good judgment. I mean she'd never give a stranger money, or walk across the street without hanging onto my arm ( she has macular degeneration) or wander off, and she's not mean (yet). But she does have her bad days and her worse days if you know what I mean.