How do you tell your loved one that they have Dementia?

How do you tell someone that they have dementia? Especially when they believe that they do not. Is it best to get a prognosis from the Dr ? And if so what kind? My family, whom doesn't spend 24/7 w her like I do don't believee me BC they aren't seeing her forget or see her walk out of the bathroom w just her diaper on to the kitchen table. She can carry on a conversation just fine. But I need to tell her 10 times a day not to bring a glass into rooms w tile. Phone rings. I have caller id on TV screen. She tells me every time it rings and every time I tell her the same thing. I know its ringing. I can see who's calling. If I want yo pick it up I will. This happens 10 times a day..... I don