There is a schematic available as one of the product photos on the page you linked:
This shows that of the three contacts associated with each relay, the center one is the common connection, one of the others is a normally-open contact, and the third is normally-closed.
Which of the actual physical pins is NC and which is NO isn't obvious from the photos and schematic, but there may be some indication in the silkscreen that isn't visible in the photos. In any case, it would be really easy to check by, for example, just hooking up a desk lamp or something through the relay on one or the other side.
You could equally well (and more safely) use a multimeter to measure the resistance from either side to the center contact and determine which is NC and which is NO.