I personally never see the rats, if you see them than you are already infested. We only see tunnel holes and the dirt piles as well as food consumption increases rapidly.
We don't have rats (that I know of) but we do have mice and sometimes my mom would put the bags of dog food in the garage and there would be a hole in the bag!!! We would also find dog kibble in our ski boots. Although recently we had a bag of food out there and they didn't touch it so must have vacated