To answer this question, any good rat exterminator will ask you to remember three words: food, shelter, and openings.
This is all what a rat infestation requires. If you have food that you do not properly cover, some spilled on the floor, stashed in dark corners by the broom, and you have an attic, shallow flooring under the closet, or crawl spaces of other kind, you can expect you have a rat infestation, even if you don’t see any.
The last thing that rats need to start treating your building like a hotel is openings. No matter how clean your building is, or how covered your food is, if you have openings big enough to fit a few pencils horizontally, you will have rats.
On the other hand, if you don’t clean your house or dishes but there are no access points for rats to use, you are safe.