You can test for amyloid, yes. The most common method is a PET scan using a tracer (ingestible marker) that sticks to amyloid and "lights up" in the scan. However there are new blood tests that works fairly well, and are getting better (but from personal experience are not as accurate as a PET scan).

You would get a prescription if you meet the criteria. First, you would need to have abnormally high amyloid levels but without any other signs or symptoms of Alzheimer's (like cognitive impairment). This is because this drug targets only the first stage (amyloid), but not any progressive stage. You would also need to meet some other criteria to determine that you are both eligible and a good candidate for it to work (e.g. no history of strokes or other brain injury). The drug is also at the moment not covered by any insurance, so you would be paying quite a bit, however this will likely change in the near future.