How Can Cognitive Impairment Be Diagnosed?

I am beginning to be very concerned about my wife (age 65) possibly having serious cognitive issues. My mother developed Dementia in her later years (she died age 92) after living a very active and fully independent life, so I am familiar with many of the issues. In my wife's instance it is a matter of her denying she ever said something which is critical to our moving forward in complex arrangents involving 'downsizing' and selling cars, our home, etc. and going into an RV lifestyle. Obviously, if it is just her saying one thing and I another - offering different views on something we both feel very strongly about - without a 'tape recording' or third party present to validate one version or another, it leads to very ugly fights and disagreements. I sometimes think I am destined to 'lose my mind.' Are there any simple on-line 'Memory Exams' or similar diagnostic tools that we both could do that might shed some light on what may be going on? Meanwhile, I fear that our marriage is in trouble and we are going to have a very bad scene here. Thank you.