B is for Blower Door

This is the way to test for air leaks, to find them, to verify they have been sealed and to determine how much air sealing to do. The average home owner is not going to pick one of these up at the local hardware store.
How tight should the house be? Most energy programs and standards look to see .3 air changes per hour or less. The standards are tightened up every few years as the research is improved.

You can air seal a house up, but please consult with a professional auditor who has the equipment and knowledge to test and direct your efforts. A professional auditor can use Infrared imaging and smoke to locate and diagnose leaks and areas to insulate in the house with a blower door.

WARNING!
It is important to get professional help if you do extensive air sealing. Tightening up the house can cause dangerous, even fatal levels of CARBON MONOXIDE to build up in the house. Professional auditors are trained to test for proper air flow, combustion safety and assess the ventilation needs of the building. Your house has to breathe, but needlessly wasting energy is letting the heat leak out. Tightening the house can result in poor indoor air quality and adversely affect the health of the occupants.