An early sign for my Mum's Alzheimer's, though I didn't make the connection until much later, was that she no longer had the concentration to read a book.
On one holiday she bought a magazine at the airport, then just sat turning the pages without actually reading anything.
My Dad has vascular dementia and his short-term memory has gone. He says himself he can't remember anything five minutes after I've told him. It doesn't bother him though!
His long-term memory is very detailed still. He's told us about all sorts of incidences from his childhood, one of being evacuated at the age of 8 with his army family by ship from India, then through France from Marseille by train, then arriving unannounced on a relative's doorstep in England at night.
We'd taken him to see the film about Sir Nicholas Winton and the Kindertransport and that prompted his story, one I'd never heard before.