There are now tests you can take to see if you are at risk for Alzheimer's. I took one such test and was found to be at 'moderate risk' although I did say I had a relative with it but she got dementia when she was 90! Not sure if that counts.
Anyway, no test can tell you for sure - not even close. But here is one test if you want to check it out
http://www.nutritionaltest.com/freealzheimer.html
Does anyone know of any other tests?