Heres the way to do a check of an area:
If you still suspect sod webworm activity but are unable to find the larvae or their frass, use a soap disclosing drench. Simply mix up two gallons of tap water with two tablespoons of liquid dishwashing detergent. Sprinkle this mix over a one square yard of the affected turf. Within a couple of minutes, the flesh-colored, spotted larvae will wriggle to the surface. If you get 10 to 15 larvae in a one square yard of turf, treatment is warranted.
I guess you have to suspect or actually see the damage occurring so if you are looking for a way to generally say there is a problem and you don't see one I would say you don't have a problem. When there is a problem its real visible.
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