You have a cartridge filter, which does [edit] NOT [/edit] get backwashed. Instead you clean the filter by turning off the pump, opening up the filter housing, removing the cartridge, washing it off throughly with a garden hose, and then putting everything back together again.
As guamguy said, the filter pressure is the best indication of when the filter needs cleaning. You should have noted down the filter pressure when the system was first installed and clean the filter when it goes up by about 1/3 from there. If you didn't note the starting filter pressure, you might as well clean the cartridge now, and then note the filter pressure when the cleaned filter is first used again.