At the ER, all they could do would be to take a CT
of the brain to check for a bleeding
stroke, which shows up immediately. An ischemic stroke (from a clot traveling from AFib heart to the brain, e.g.) wouldn't show up until a day or two later, as changes occur to the dead brain tissue. Still, if blood flow is restored by itself, and there is no permanent damage, then nothing would likely show up anyway.