Early Stage Dementia? How was it diagnosed?

It must depend on the person.  Best to get everything like paperwork and wherever they'll live in order now.  I noticed one parent had problems maybe 4 years before diagnosed, but due to personality quirks, everyone ignored; now real bad.  The other was never diagnosed, but exhibited maybe 7 years after the 1st one. 

Losing things seems to be a big one, not being able to focus, letting bills go unpaid or paying late, ignoring checkbooks & banking, bad checks, lost when driving in day to known places, rages, letting house/apt. go: not cleaning, not taking out old food, eating out of date food, burnt pans, etc., any hygiene they normally practiced but now don't do. 

It's not hard, but you need to use your keen eyes (a little snoopy, but you have to do it) to really see what's going on (or what's not going on) and your keen ears to listen and really hear what's being said or not said. 

Sometimes a cry for help comes in different ways than we expect in what a person says or doesn't say and could be through what we see.  If we're listening good (and looking), we hear (or see) it and act.

I began to suspect something was “up” with my mother when she began telling me a man in her apartment building was “stalking” her. When she’d see him out walking in the morning, she’d claim that he was spying on her even though she lived on the 4th floor. It got worse and worse. He was breaking into her apartment and stealing things. Putting surveillance camera in her lamps. Then, one day I walked into her apartment and was met with a blast of heat. She’d turned her oven on and opened the oven door. There was no reason for it. Eventually, she was admitted to the hospital with a raging urinary tract infection. Her behavior was combative and they had her tested by the neurologist on staff. She failed the testing. The social worker spoke with me and told me they felt she could no longer live on her own. She helped me find a wonderful facility for Mom. She lived there for three years until she passed at the age of 95 in 2016.

I had a suspicion there was "something" going on and finally convince our primary doctor to refer him for testing. (our doctor had previously shrugged off my requests for a referral and did a MME and said he was fine.
I have read that many people have hidden signs for as long as 10 years before they are actually diagnosed. If you think about all the "little things" that just did not seem right or just a bit off the picture comes into focus.
My Husband was in his early 60's when diagnosed and died 12 years later.