What can help detect a carbon dioxide buildup?

Answer 1

A modern battery-operated carbon dioxide/monoxide detector is the best way to detect a gas build-up. Historical note: A caged canary used to be taken down coal mines. If the canary collapsed, there was carbon dioxide present at a dangerous level to the coal miners.

Answer 2

Carbon dioxise sensors or detectors fall into two categories:

• Infrared gas sensors (NDIR). These work by passing a beam of infrared light through a gas stream. Carbon dioxide is absorbed from the stream at a know rate depending on its concentration. changes in the light power are converted to a concentration readout.
  
• Chemical gas sensors relate the chemical resistance of strips of chemicals on plastic which vary as the carbon dioxide concentration in the air.