Wet grass: Sometimes a home's yard might be wet when you arrive, which helps give the grass an attractive sheen. Take a moment and make sure all areas of the lawn are wet. Dry patches could indicate problems with automatic sprinklers.
Mood-setting music: It's pretty common for real estate agents to play light music during an open house. However, if you're near an airport or a busy highway, ask if the stereo can be turned down to see if the noise is bothersome.
Clean carpets: If the carpets are fairly new and in good shape, the owners probably had them cleaned before the open house. If they aren't nice, sellers won't want to put money into cleaning them since a new buyer will probably replace them anyway. If you think you would keep the carpets if you bought the home, inspect them closely. Any stains still visible after a cleaning probably won't ever come out.
Plant patrol: If you see potted plants stationed strategically outside a house where flowers and shrubbery would normally go, be a little suspicious. Sometimes this is done to give the yard a fresh look, but it could also indicate soil problems or a lack of sunlight.