In the US where I live, professional mold testing companies have big expensive machines which supposedly count spores in the air. The results always come back negative, unless the space is visibly super contaminated. This is because mold is usually in the walls and these services are called in after a water issues to absolve landlords of financial responsibility for the extensive work and expense it would take to actually solve mold problems. It’s a nasty business.

We had our place professionally tested, got negative results, then tore the place down to the studs anyway (eventually, after the Petri dish tests described above) and found all sorts of mold.