Perhaps checking if the acceleration has changed in the last .25 seconds? You could test this by giving the screen a red area labelled "TOO MUCH MOTION" unless it's still. You could also let the user determine the threshold for a) how much acceleration change is OK and b) how long a time window to look for (i.e. LESS THAN x ACCELERATION FOR y TENTHS OF A SECOND)
Perhaps checking if the acceleration has changed in the last .25 seconds? You could test this by giving the screen a red area labelled "TOO MUCH MOTION" unless it's still. You could also let the user determine the threshold for a) how much acceleration change is OK and b) how long a time window to look for (i.e. LESS THAN x ACCELERATION FOR y TENTHS OF A SECOND)