We purchased our house in 2005, and I have no idea whether the previous occupants ever had the air ducts cleaned. So, how exactly do you know if the ducts need cleaning? Is it as simple as taking off one of the vent covers and looking inside?
easy way with exposed duct in basement would to remove the end cap on the return side and look inside. the return will get caked up with dirt more then the supply. A indication that ducts have been cleaned would be little black plugs on the duct in a bunch of area where they would drill holes to do work and inspect.
if you shut the furnace down remove the fan cover and run your index finger inside the rounds of th squirrel cage...if it is lite dust your clean.remove one of your return grills and check it and if it is a straight drop to the filter take a white rag and roll it in a ball and drop it down the duct...and see the results.a clean house with minimal carpeting should be clean . what dirty ducts are.... next time your in a food store or those bid drug stores you will see black tracking on to the ceiling tiles...that means the dirt is going past an already packed filter and traveling thru the system.