See below. Look for any change of personality, memory loss and loss of functional activity such as bathing, dressing. The new onset of dementia varies by age. For down's syndrome, it is best diagnose by looking at the clinical symptoms - behaviorally, functionally and cognitively - almost same way as someone diagnosis for alzheimer's disease.
Similar to Alzheimer. Any deviations from the baseline functioning -- changes in memory, affect, adl, etc. Are usually investigated. The diagnosis of dementia in Down syndrome is somewhat similar to the diagnosis of alzheimer's disease, however, the initial disabilities of Down syndrome should be taken into account.