Whether the space is occupied is not important, the issue is whether it could be. If not, it is considered a non-permit confined space. The potential to contain a hazardous atmosphere as defined by OSHA should be an easy element of the definition to determine. In the broadest interpretation, almost all confined spaces have the potential for a hazardous atmosphere. To classify a space as not having a hazardous potential, we need to be certain beyond any reasonable doubt that an atmospheric hazard does not and will not exist in the space while we have entry personnel in the space.
Oxygen concentration is usually fairly straightforward.
Investigate any variance from normal atmospheric conditions. Any measureable quantity of flammable gases or vapors should be considered a problem and investigated. Any detectable concentration of toxic materials should be investigated thoroughly. The general rule that OSHA provides on dust does not provide adequate information to make safe choices concerning all combustible dusts and all conditions. Some dusts may not obscure your vision at five feet and still explode, others may obscure your vision at considerably less than five feet and not explode.