Observe whether the toilet is draining slower than usual when flushed.
Check to see if water is backing up into sinks, toilets, tubs or washing machines.
Listen for any unusual sounds such as "gurgling" when you flush a toilet or turn on the faucet.
Check the soil around the cesspool. If there are noticeably soggy areas, or the grass is a deeper green color than surrounding areas, the cesspool is likely full.
Be on the lookout for depressions in the soil.
Be aware of any unpleasant odors present either in the house or outside the house.