Usually they can tell by your symptoms, vomiting, dilated pupils,,severe headache, confusion, extremely sleepy, A CT or MRI scan. Treatment is usually just get plenty of rest, stay hydrated, aspirin cold cloth--not much really
It depends on the severity of the impact that caused the concussion. Concussions occur when your head is struck hard enough to literally dislodge your brain and cause it to impact your interior cerebral casing - hit hard enough can result in brain damage. Typically concussions might result in some slight edema - but our brains are pretty well protected - and pliant - so recovery is usually complete - as long as you have a physician examine you - if he or she detects some impairment in assessing basic functioning tests - they may order an MRI or other scan appropriate to your injury. What they are most cautious of is any extensive edema (swelling) or possible bleeds - which might then require surgery to repair.