In hindsight it should have been obvious from the start
My mum's dementia was of sudden onset but her formal diagnosis was a laughable 18 months after she'd been looked after 24/7 by private live-in carers. In 2007 she was admitted to hospital with a rare and life-threatening condition that, among other things, inflamed and damaged the small blood vessels in her brain.
In Jan 2008 my brother and I got her home from hospital with private carers. SS and CMHT had been keeping her there for no other reason than they had decided she needed to live in a NH and hadn't got a suitable place. We kept saying we wanted her to come home; they said she couldn't manage to live alone any more and that it was impossible to get carers to live in. "We've never heard of that. Maybe in England, but you won't find carers like that in Scotland" was their mantra.
My brother and I found it hard to engage with people who thought they knew best and that we had no place in the decision-making process, even though my brother has welfare POA. The hospital doctors and nurses refused to tell us when she was ready for discharge. They were sarcastic to me and treated me like a pest, getting in their way when they were so busy. So busy providing our mum with bed and board on the NHS tab, when she was well enough to go home, but they wouldn't admit this, presumably because the hospital SW had told them not to? I asked to have a meeting to have input into her discharge plan. What's that? they said.
The arrogance of the authorities was unbelievable. We eventually decided to tell them we were taking her home. The nurses couldn't pack her case fast enough and then admitted that she could have been discharged 6 weeks ago if there had been somewhere suitable for her to go.
The point of rehearsing that sorry saga is that after all this excessive 'care', once we decided to bypass what SS decided was best for her then we found ourselves completely alone in managing our mum's care. She was immediately taken off the SS books. Her GP monitored her physical recovery but refused to accept that she had dementia. We ourselves didn't know how much she would recover. Physically she was still convalescing, very thin, learning to walk again, and plagued by hallucinations and paranoia after months living on a hospital ward. We hoped that after a few months she would be physically well and could then perhaps have paid help in the house as required, maybe not needing a live-in carer any more.
When 7 months later I eventually got SS engaged again in order to get Direct Payment funding we were fortunate to have an excellent SW with mental health nursing experience. It was she who confirmed that our mum had dementia, that she wasn't going to recover mentally, and that she would slowly get worse. The official NHS diagnosis came in 2009 when the psychiatrist assessed her as not having capacity with regard to legal and financial matters. She had wanted to change her Will and the solicitor called in the psychiatrist to check whether it was OK to take her instruction.
That's the very long answer to "what gave you the first clue..." The short answer is that from the moment our mum returned to her own home she was entirely passive about all domestic arrangements, personal care and anything that involved decision-making. She resented the presence of the carers but expected them to do everything. If they tried to engage her in cleaning or meal preparation as an activity to engage her mentally, she would always refuse. She just wanted to sit in her chair, or preferabaly take to her bed, and be waited on.
Prior to her severe illness she had been, we believe, mentally ill for a few years but our father covered this up as nerves because she was over emotional and mustn't be upset. When he died this got worse, but we put it down to the stress of grief and having to tackle the mountain of administration that comes with the death of your partner, especially when he had been the one who dealt with finance, utilities, insurance, and so on. She completely panicked when faced with this responsibility. Therefore some of her oddness afterwards was a continuation of previous behaviour. We didn't give it the label of dementia until probably a year after she had suffered catastrophic inflammatory brain damage and consequent dementia of sudden onset.