The two sentences have different meanings, and the simple present version would be incorrect in this context.
Present continuous generally means something that is currently true, so if you have a patient and they are suffering from a concussion, it means right now. So the question, "How do we know when someone is suffering from concussion?" means, "How do we identify that a patient is currently suffering from concussion?". The answer to this question is a list of symptoms that present when a person is currently suffering from a concussion.
Simple present generally means something that happens regularly, from time to time, and not necessarily now, so if you have a patient and they suffer from concussion, it means from time to time. So the question, "How do we know when someone suffers from concussion?" means, "How do we identify that a patient regularly suffers concussions?". The answer to this question is a list of symptoms of people who regularly suffer concussions.