6. Stains of grease
Because of the oil on their skin, they will leave a trace behind them.
7. You start hearing unusual noises
They can be silent creatures, but you are going to hear them in your house. For example, termites will make a lot of noise, by banging their tiny hands into the wall. If you hear something between the walls or in the ceiling, then you can also have a rat or mice problem in your place.