There is no single test that proves a person has dementia. A diagnosis is made through a complete assessment that considers all possible causes. One of tests used is the mini-cog(nition) test. The mini-cog is one of a number of memory tests that have started to pop up in routine check-ups of older patients. One part of the test involves drawing the face of a clock.
Clock face drawing has been validated in multiple studies and is still generally respected as useful. It tests integration of multiple areas of the brain, so it has high sensitivity – in other words, it can show up problems before other tests do.
Even if a person hasn't seen a clock face in 20 years, it is something that everyone learned when children and was drilled in primary schools and is therefore likely to still be intact. That said, it is only a screening tool and only part of a range of tests used to actually diagnose the person.
During the mini-cog, a person is asked to complete two tasks:
Remember and a few minutes later repeat the names of three common objects
Draw a face of a clock showing all 12 numbers in the right places and a time specified by the examiner
The results of this brief test can help a doctor determine whether further evaluation is required. While tragic of course, personally, I find the resulting drawings by people with dementia below both haunting and rather beautiful.
By the time today's children – who hardly use analogue clocks – are old enough to have to have such a test, hopefully, medical science will have something a little more, well, scientific...