Does seeing one cockroach mean an infestation?

Not necessarily. One cockroach may have wandered indoors, but it can also be the first visible sign of a hidden population. The species matters: some cockroaches mainly live outdoors, while others breed inside buildings. A single sighting cannot tell you how many are present. (University of Minnesota Extension: Cockroaches; UC IPM: Cockroach identification and management)

Look for repeated sightings, small cockroaches, egg cases or droppings around the same area. Monitor likely hiding places with suitable sticky traps and note where insects are found. Repeated activity is a stronger reason to investigate than one isolated sighting, especially around kitchen cupboards or appliances. (University of Minnesota Extension: Cockroaches)