As another top level comment describes, there are very inexpensive hardware store mold testing kits which are essentially just petri dishes you leave open and then mail in for testing.

Also if it is indeed mold, the source is typically some source of moisture. You want to rule out roof leaks and plumbing leaks, and bear in mind that water is incredibly non intuitive and creative in getting from point a to point b.

Another major source of humidity is the human body. Between sweat and respiration, we emit quite a bit of water. This goes into the air and into the bedding and requires ventilation to dissipate. Tightly sealed under-bed areas prevent the mattress from breathing. You might consider stripping the bedding and lifting up the mattress to see how things look.

Consider the rooms' ventilation system. Are the filters new, are the ducts clean? Hiring a professional to check the HVAC system is good routine maintenance.

A final source of moisture leading to mold to consider is condensation. Anywhere warm air meets a much colder surface leads to liquid water and mold. This can mean cracks and poor insulation in exterior walls during winter, or the air conditioning parts during summer. I've seen this combine with the bed-human moisture where under a bed against an exterior wall in a poorly insulated house was staying cold. The bottom of the mattress was moldy.

Mold is insidious. You often need to get into the mindset of a detective, or hire one, to find it. Good luck.