Originally Posted by
hailstorm
Even if there's no sign, you'll most likely see a lot of shoes and shoeless children in the area to tip you off.
Agreed.
In the case that you might be the first one there, look out for signs of 'virtual tatami'; a delineated area with a change in colour/textured flooring, or a line or ridge in the floor marking a boundary. Often there will be seats or benches on the perimeter.