How can I tell if my mom in law is mentally ill or has dementia?

She has had outbursts of anger, rage, screaming fearfully. she can't follow a line of conversation for long. She sits and moves papers from pile to pile in her room and thinks she is cleaning. She will read junk mail for hours and hours. She won't eat sometimes unless I make her. She was always mean to most people in her life and a loner. She pushed me down when I was pregnant, told me to get an abortion, sabotaged our marriage, and the marriages of her other son. she has always wanted control over everything. She seems nicer now that she is older and does have some dementia, but she seems paranoid and has major outbursts of anger, chasing me in her wheelchair saying she wants to kill me and her son (who lives with her). I am sooo tired of it at times.