Almost every steep thatched roof in Ogimachi is a house somebody lives in. Nine buildings in Shirakawa-mura are open to visitors, and this page says which of them are open on the day you are coming, what it costs and when they close.
The village puts it plainly on its own site: 世界遺産集落は住民が生活しています — people live in the World Heritage settlement — do not walk into someone's grounds or fields, look in through a window, or open a door. There is no published list of which houses are private, because it is the other way round: these nine are open, and everything else is a home. The famous view of the roofs is from the Ogimachi castle-ruin viewpoint on the hill, which is free, has no gate and no closing time.
The village also asks that visits to the settlement happen between 8:00 and 17:00, outside light-up evenings, and that is why the public car park keeps those hours.