Gnaw Marks and Holes
Researchers still don't really know why, but for some reason, rats love to gnaw on things. And while this is an unpleasant habit that could ruin some of your stuff, it's fortunately an observable red flag for a potential tenant to suspect an infestation. If you spot chewed-up electrical wires, this could be a sign of rats or mice. Rats love to gnaw on all kinds of material, from plastic to wood or even metal. If you spot little gnaw marks, there's a good chance rats roam the place at night.
Holes in or around the property are also important giveaways that indicate a possible infestation. If you find burrows in weedy areas and garbage areas near the property, this is a bad sign. And don't discount even small holes in walls and floors. Small insects, of course, can enter rooms this way, and even rats can squeeze through holes less than an inch wide by collapsing their skeleton [source: Sullivan]. Of course, small holes are common, especially in older buildings. But you can point them out and ask the landlord to seal them off.