A petri dish test is used to detect mold spore in the air. The test is not terribly useful for mold on a surface. It is a valid test if you or someone in your family has an allergy to mold spores and wants to avoid breathing the stuff, since all you need is pass/failó you don't need to know how many parts per million, just is it there or isn't it.
If you want to try the petri dish method, go to Home Depot and buy a "Mold Armor" test kit. Cost is about $8. Much cheaper than hiring someone to test for mold. Problem is, if mold is present in the air, where did it come from?
The best way to "test" for mold on surfaces is to not test at all. Use your own senses. Mold isn't invisible; you can see it with the naked eye, so LOOK for mold, in cracks and crevices, areas prone to dampness, and areas that never get direct sunlight. Wear a painter's dust mask while looking, so that you don't accidentally inhale any mold that you accidentally disturb.
Engineering: Finding complex solutions to simple problems you didn't even know you had.