The furnace and air conditioning can be checked my turning it on. Depending on the season and weather you might only be able to turn on one. Check the air flow at the vents. Listen for any strange noises. Check the physical condition of the furnace and air conditioning. Is it damaged, dirty, or rusting? Look for the serial number and use Google to find out how old the unit is. These systems usually last about 15 years. The furnace has a panel that can be removed to check inside and check for damage, dirt and rust. Ask the home owner for maintenance records. We recommend yearly service. Check the filter.