Mom seems to have symptoms of dementia. Is there a test/blood test? How is it diagnosed?

Docs make best guess based on cognitive testing and behaviors. The only sure way to know is upon death with an autopsy.

Wife was diagnosed with a CT scan. Hers is FTD with aphasia. And getting worse.
I was told the type of test would depend on her scores from the tests like drawing the clock.

You can take Mom to a neurologist and he will evaluate Mom and do what testing he feels necessary. There is one called an EEG that tests the brain waves.

Blood tests really don't show dementia; you generally need a full cognitive workup. You also need to rule out a lot of metabolic stuff such as hydrocephalus, sodium imbalance and UTIs. Tell us more and we can give you better information.