For isolation and identification of Aspergillus fumigatus from soil you can use Czapek Dox media. For keys and subspecies description you can use the book of Raper and Fennell "The genus Aspergillus".
we can use air sampler : i have experience with it in chemotherapy wards of cancer hospital: as you may agree isolation is not difficult and ID can be done by conventional methods
Off-topic: be very careful, working with this MO could be very dangerous, due to the small size of the spores. A colleague of mine showed me a good tip - always when you have to work with A.f. colonies, dive your inoculating loop in sterile water to cool it down (after burner, before picking the cells).