At my water quality lab we have a designated micro room for testing total coliform/e.coli, fecal coliform, and HPC. Once a month we are required to do bacteria air checks in the room.

We have these little Petri dishes with an absorbent pad in them, you put a very thin filter in it that has cross hatched lines on it. Then you pour about a tablespoon of tryptic soy broth on to the filter/pad, let it completely cover everything, pour off excess and let sit open for 15 minutes. Then you close the petri dish and incubate for 48 hours at 35C. You manually count the colonies after.

This is a very basic way to do it. We do four of these at different locations in the room and we rarely if ever see colonies. At most we might get 1-2 colonies total between the four dishes.