How do you find out what size sand filter you have?

Usually the top of the sand filter is above the water level of the pool, so all you have to do is unscrew the top and look in. However if the filter is below the pools water level you will have to cut of the water coming from and to the pool before you do. who added the sand? If you did - did you follow the manufacturer's instructions as to how much and what type of sand to use? Instructions tell you - - SPECIFICALLY - - how many pounds of # 20 silica sand to use.