How can you really tell when your loved one is losing there memory?

If I ask her any question, The answer I always get is, " I don't remember or I have forgot or I am not sure". In the last 3 years i have NEVER got a positive answer from her yet.(I really love her to pieces, even after 43 years marriage.) She is 63 years young & went to Girls Grammer School and had very good school reports throughout her education. Any thoughts or suggestions that will help would be most appreachiated.
Many Many thanks for listening.
connormax2