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. Rhubarb is a hardy perennial that is not usually seriously affected by insect pests, however, occasional attacks of the rhubarb curculio do occur. The leaf stalks of the rhubarb may show exuding sap and partial decay from late-May through early summer due to the feeding and egg laying punctures of the rhubarb curculio.