Some people have suggested that suffering a head injury early in life increases the risk of developing Alzheimer's later in life, but the evidence is mixed for that proposition. It appears that the more education one has the less likely one will develop Alzheimer's. However, it's not clear what that result implies. Alzheimer's is degenerative, so once the damage begins it will continue until the person dies. There is no cure.
Diagnosis is done by asking the individual a series of questions to test their short term memory.
Also, Alzheimer's can be diagnosed by ruling out other possible causal explanations for a memory deficit, such as a tumor, or stroke. The only way to know that a person has Alzheimer's for sure is to do an autopsy after the person dies. If the diagnosis can be made, then Alzheimer's is already very far along.