First of all, if you see mold growth you should look for the water or moisture source and try to stop or prevent it. By preventing an accumulation of moisture or water, you prevent mold growth. Where there is minimal or no visible mold growth, but there is a musty or moldy odor, the investigation of where it's coming from becomes a little more difficult. Again look for any area of moisture.
If there is a concern about elevated mold spores, then air sampling maybe required. A trained and experienced environmental consultant should do the sampling. Sampling can also be very expensive. Depending on the environmental consultant, the rates for services can range from $75.00 to $300.00 per hour plus expenses and lab costs.