The easiest way to determine whether you have rats or mice is to see the rodent. Mice are much smaller than any species of rats. It’s similar to comparing your thumb to your entire palm. As a result, rat nests are typically larger, too. But if you can’t see any pests or find their nests, telling them apart may be difficult without a professional inspection. If you find droppings from the invading rodent, take a glance before you clean them up. Rat droppings are larger and rounder than mouse droppings (which are pointed).