Backwash the sand filter by shutting off the main pump at the control panel and following the manufacturer's instructions for your specific model to complete the backwashing procedure. After cleaning, write down the pressure reading on the filter, and the date.
Look at the pressure reading for the filter on a daily basis and compare it to the original one you wrote down. When the reading increases to between 8 lbs. and 10 lbs. over the pressure recorded after cleaning, it's time to backwash again or to replace the sand. Go to Step 3 to know which.
Compare the date that the pressure has risen 8 lbs. to 10 lbs. with the date you wrote down at the previous backwashing. If the dates are within two weeks or less of each other, it's time to replace the sand in the filter.
Run your finger over some of the pool elements, such as the inside of the filter and the pump assembly. If your finger feels greasy or you see visible residue, the sand is not filtering properly, and you should replace it.