bass strait

Victoria's Surf Coast is uniquely positioned with a broad swell window to the southwest, however incoming swells are refracted as they round Cape Otway. This narrows the apparent swell direction at the coast, so when viewed from shore the majority of swells appear to arrive from the south. So how can you tell if there's a westerly swell or a southerly swell running through Bass Strait if they all appear to be arriving from a similar direction at the coast?