Taken from the NFHS Softball Umpires Manual:

Appeal at the Plate

If a runner misses home plate and the catcher misses the tag, the umpire should hesitate slightly to allow the players to finish the play, either the runner reaches to tag the plate or the catcher reaches to tag the runner. If no tag is made, the umpires should declare the runner SAFE. If a proper appeal play is made by the fielder, by either touching the runner or the plate with the ball prior to the runner touching the plate, the umpire should then declare the runner OUT.

There are two reasons why we (NFHS) advocate this procedure: 1) We do not want to tip either team that the play may not be over, and 2) a runner is assumed SAFE until put out. If proper appeal is not made, the runner is SAFE.