A few roaches versus a real infestation
An infestation means cockroaches are living and breeding in the unit — you see them regularly, including during the day, spot droppings or egg cases, or notice a musty smell — not just one roach that wandered in.
Seeing a single roach once in a while, especially in a warm climate or a large building, does not always mean an infestation. The signs of a real problem are roaches you see often, roaches out in daylight (a sign the population has grown crowded), small dark droppings that look like ground pepper or coffee, tan egg cases, and a lingering oily or musty odor.
The more of these signs you notice, the more serious the problem — and the stronger the case that it needs professional treatment rather than a can of spray. If you are not sure whether what you are seeing counts as an infestation, documenting it with photos is a good first step, and a quick conversation can help you gauge how serious it is.
This is general information about California tenant rights, not legal advice. Every situation is different.