P0440 is a small evap leak......basically, all you can do is a visual inspection of the system....unless your see a bad line, bad connection, smell fuel, etc., there is not much you can do....
The PCM tests the system like this: it closes the vent valve, and then opens the purge, to pull a vac on the system, then it closes the purge valve and the PCM monitors the vac that should hold in a closed system(via the pressure sensor)....if the vac drops too fast in a specific period of time, then a leak is detected......
The first thing that should be done with a P0440, is inspect the gas cap.....does the contact surface of the fill pipe look rusted? Does the seal on the gas cap look cracked/degraded?
More often than not a P0440, is caused by a bad gas cap....so replace it and clear the codes...if it comes back, and a visual inspection of the whole system shows nothing obvious, then you are just wasting your time....the fasted way to diagnose the system is bring it to a garage....they have a bidirectional scan tool, and a smoke machine......basically, they seal the system, and pressurize it with smoke....then you look for smoke(with a car on a lift, this is fairly easy to do)......when you see smoke, you have found the leak......