Not long ago, the only way to know if someone had Alzheimer’s disease was to examine the brain in an autopsy.

That is changing — and fast — with brain scans and spinal taps that can detect beta amyloid, the telltale Alzheimer’s protein.

There is a blood test on the horizon that can detect beta amyloid, and researchers are experimenting with scans to look for another protein, called tau, also characteristic of Alzheimer’s.

As this sort of diagnostic testing becomes widespread, more people who fear their memories are slipping will face a difficult question: Would I really want to know if I were getting Alzheimer’s disease?

“This is a new era, and we are just at the precipice,” said Dr. Gil Rabinovici, a neurologist at the University of California, San Francisco.

A positive test could help you get your affairs in order and plan your future. And a drug company, Biogen, claims to have the first treatment that may slow the course of the disease if begun early enough.

Health insurers are prohibited by law — for now, at least — from denying coverage if you have Alzheimer’s. But there is nothing that prevents long-term-care and life insurers from denying you.

Will your friends stay with you? How about your spouse? What would it be like to live with the knowledge that you will eventually be unable to recognize your family, or even to speak?

For some who have been given diagnostic tests, those questions are all too real.

When Dr. Daniel Gibbs, 68, a neurologist in Portland, Ore., noticed his memory starting to slip, he wanted to know if it was Alzheimer’s. He had seen its damage all too often in his patients.

So he received brain scans for beta amyloid and took cognitive tests. He knew that in people like him with mild memory problems, the combination can cinch a diagnosis.

The result? He was in the early stages of Alzheimer’s disease.

Now he worries about his future. Alzheimer’s is “an ugly way to die,” he said. He has told his family that if he gets something like pneumonia, they should withhold treatment.

The sort of testing Dr. Gibbs had can be expensive, and diagnostic brain scans usually are not covered by insurance. The tests are not for the worried well whose memories are intact.

But they are available at some medical centers to those with mild memory problems.

Generally, even people without memory problems who have amyloid plaques in their brains are more likely to progress to Alzheimer’s, said Dr. Ronald Petersen, a neurologist at the Mayo Clinic in Rochester, Minn.

But not everyone does progress. Even when they do, it can be years before there are symptoms.

Outside of research studies, Dr. Petersen said, “we do not do amyloid scans on clinically normal people, because we don’t know what to tell them.”

Dr. Rabinovici, of U.C.S.F., sees people who are distressed by their memory problems, knowing something is wrong and unable to get an answer from their doctors.

“Often doctors can’t definitively tell them if their memory loss is related to aging,” he said. “A lot of times, doctors dismiss it and tell patients: ‘You are fine, it’s normal. You are 75 or 89 and depressed. Why not try an antidepressant?’”

Before he offers diagnostic testing, Dr. Rabinovici sits down with patients and their families and asks how will they feel if the test is positive — or negative.

Most who receive positive diagnoses have told him that after the initial shock, they did not regret being tested. “It ends the diagnostic odyssey,” he said. “It ends the uncertainty.”

Dr. Jason Karlawish, an Alzheimer’s researcher at the University of Pennsylvania, did a formal study to gauge patients’ responses to learning that they had elevated levels of amyloid in their brain.

He did not see catastrophic reactions to the bad news. No one died by suicide.

Instead, many said they were taking steps to slow Alzheimer’s, putting their faith in healthy diets and exercise although no lifestyle measures have been shown to have an effect.

But some were not so sure getting a diagnosis had helped them. “You’ve now told me something about my future,” Dr. Karlawish recalled one patient telling him. “I can’t unlearn this.”

For some, the diagnosis has unleashed a storm of emotions.

Wallace Rueckel, 75, of St. James, N.C., worries about what will happen as his disease progresses. He has been reluctant to let people other than relatives know he has early Alzheimer’s disease.

“I don’t want people to feel sorry for me,” he said.

Jay Reinstein of Raleigh, N.C., 58, learned he had early-stage Alzheimer’s disease in March of 2018. He stepped down from his job as an assistant city manager in Fayetteville, N.C., later that year.

“I was numb,” Mr. Reinstein said. “I loved working. Work was my life. That was my identity.”

And he was not the only one left reeling by the diagnosis.

“My wife is not doing well with this,” Mr. Reinstein said. “It has really taken a major toll on our family. I have become depressed.”

He worries what his life will be like in a few years. He worries that friends will fall away.

Mr. Reinstein has changed his diet, and now he exercises. He and his wife have made a will. He decided to become active in the Alzheimer’s Association and to do what he can to destigmatize the disease.

He is trying not to let the diagnosis take over his life. “I don’t want to be defined by the disease,” he said.