Roach Infestation Question

Hello! How do you know if an infestation is completely gone?

I saw 2-4 roaches recently, 2 of them escaped so I'm not sure if they were 2 unique roaches or 3-4. I live in a pretty small studio apartment so I never saw any droppings, dead ones or eggs. No musty smell either. Besides the 2, I haven't seen any scurrying when the lights turn on. I've stepped up my cleaning and did some DIY treatments.

How do I know if these treatments work if I never saw any other signs of an infestation? Is it possible they've just retreated back into the walls?

Thanks!