Lay down several sheets of newspaper outside in a well-ventilated area. Completely cover the area you will be working in.
Point the spray nozzle at the newspaper and depress. If no paint comes out and there isn't a hissing noise, the can is empty. Otherwise, spray the paint onto the newspaper until it's empty.
Shake the can if nothing comes out to judge whether it is empty. If you are unsure, assume there is still paint in the can.
Contact your waste management company or local dump to learn the proper way for disposal. Empty cans most likely can be placed in your regular trash. Cans with paint inside will need to be brought to or picked up by a hazardous waste facility, which might charge a small fee.