What are the symptoms of a major stroke?

Location dependent. This depends upon the location of the stroke. It could include weakness, language difficulties, sensory abnormalities and visual loss. Other possibilites include personality change, neglect of one side of the body, transient visual hallucinations, inability to swallow, facial weakness and numbness, gait disorders, tremors and spasticity. The list is not all inclusive.
Not subtle. "major" means life altering, including paralysis, blindness, speech loss (aphasia) and death. One will have no difficulty in determining whether a "major" stroke has occurred. The problem arises in detecting milder or even asymptomatic strokes.