Things to do at an open house, or first viewing, before home inspection:

1) Check water pressure.
2) Open closets/cabinets.
3) Check cell phone service.
4) Listen for ambient noise (neighbor footsteps, street noise;etc)
5) Figure out which direction unit is facing/has exposure to.
6) Learn type of heat source (forced air, radiator, baseboard – gas, oil, electric).
7) Notice the number of built in lights per room, as most properties are viewed in daylight.
8) Check quality of doors and railings. They are often an indicator to quality of construction.
9) Take measurements.
10) Open/close windows and check for broken seals (fogginess)