We classify dementia by the patterns we see in affected brain tissue under the microscope. Amyloid plaques and tau tangles -> Alzheimer's. Lewy bodies -> Lewy body dementia. Pick bodies -> frontotemporal dementia. It's more complex but these are examples of elements that help make a tissue diagnosis.

Since most people don't undergo brain biopsies while alive, the best we can do in a living patient is make an educated guess based on symptoms, and sometimes supplement with specialized imaging and spinal fluid tests.