There's some care to take here, especially depending on state licensing requirements. In some places it's particularly easy or low risk to present fraudulent results or interpretations of results on air tests so they can pitch expensive but unnecessary mitigation. National and regional chains often provide licensing cover for, and set quotas on, operators that enable some of this scamming.

One way to vet a mold tester is to go through a property management or landlords' association, even if you're not either of those things (even if you're a renter!). They often refer or have a list of vetted local contractors publicly available. Nobody likes wasting money on property less than a landlord.