Footprints and tail drags can be seen in dusty locations. Use notoxic tracking dust such as talcum powder or flour to determine if rodents are frequeting certain areas. Gnaw marks are a sure sign of rats. On wood, the older the gnawing, the darker the wood. If dogs or cats unexplainably get excited, rats are probably moving about in walls voids or ceilings.
Rats produce a distinctive musky odor.
An experienced rodent-control person can tell hte diffference between mouse and rat odors. Nests are another sign. Norway rats usually burrow, but nests under concrete slabs, in rag pile or in lumber piles are not unusual. Roof rats' nests are usually up high and are often difficult to find. Sometiems nests are similar to tree squirrles' nests, consisting ofleaves, twigs and vines.