Inspection for Fungus Gnats. 1  Inspect soil of potted plants. 2  When plants are overwatered, increasing moisture it causes conditions for Fungus Gnats to breed. 3  Look for adult Gnats flying around the plants. This would be an indication of soil under the plants as a breeding source. It may be difficult to find or see larvae of Gnats in soil. If no planters or atrium areas are found to be breeding sources inside, the gnats may be entering the building from the outside.