The floats have viewports on the deck, so a red painted portion of the landing gear actuator will be visible as up or down. The nosewheels, or flags on the nose gear arms are visible in the up position. Absence of nose gear up has to be taken as nose gear down. In some systems, it is possible to have one side up, and the other side down, so both sides must be checked to be sure. This has happened to me twice, so I always check both sides visually.