Properties holding an improbable number of people in a single census year. Usually this is several households filed against one address rather than a genuinely enormous one — the Pembertons at 1 Kenilworth Road were found this way.
Surnames is the stronger signal. A real household is rarely more than two or three families. Twenty people under twenty surnames is not a household.
Newest first, whatever their size — so a row pressed by mistake can be found and undone even after the house has been sorted out and dropped off the list above.