It's a square black signal on the top of the roof.
Visit Instructions:We don't require you to provide any specific proof of your visit, but we do request that you upload photos if possible, the more the merrier.
Ideally, you should try to take photos at the actual beacon location, but if that isn't possible (private property, terrain, etc), then photos taken from a distance are also welcome, especially if the beacon is visible in the photo.