100-hour private pilot took a trip with a passenger to a small town airport to visit someone and figured he would have known and told us if the airport had any problems. Didn't check notams before flying. Arrived and overflew midfield, noticed a couple of vehicles in the grass near the runway and X's on the ends of the second grass runway. There were X's in the grass off the ends of the paved runway and it didn't have any paint on it. Saw a spray plane with its door open in front of a hangar full of chemical. We debated if the airport was open or not and decided it was safe to land, other planes were operating there, and AWOS was broadcasting weather information and nothing about closure. Once we parked and shut down, a guy walked over and asked if we knew we had landed at a closed airport. Checked the notams then and saw that both runways were closed. He told us to move the plane off the marked parking space we were on to the other end of the airport so the spray planes could taxi through. We asked where the spray planes were taking off and landing and he said the runway. We watched a spray plane take off from the field while we waited for our ride to show up. So, the PIC didn't find out all the information possible before the flight. That's a rule and it was broken. But apparently the local custom is to keep using the runway even if it's closed. Someone told me that the FAA has some kind of anonymous self-reporting service for incidents like this. Anyone know where to find that? And who among POA would self-report this incident?