How do I know it is a rat?

Actually, mice are pretty bright, and often have a more pointed snout than rats.

Adult rats are almost always bigger than mice. Even the biggest bred-for-show pet mice are only 5” or 6” long without the tail, whereas only the very tiniest female ship rats are that small. Generally, small adult female ship rats (the smaller species) start at the size of a hamster, and large adult male brown rats are as big as a medium-sized guinea-pig.

Baby ship rats - Rattus rattus - are quite hard to tell from mice, but they have longer, coarser coats.

Brown rats - Rattus norvegicus - even as babies are a different shape from mice. They have a powerful, slightly rabbittish rump, whereas mice are about the same height all along. Brown rats have proportionately much smaller ears than mice and shorter, thicker tails. Their coats are thicker, their jaws are more powerful-looking and they do not have such a marked distinction between the muzzle and the cheekbones.

They also smell different. Female mice have no odour, but male mice have a strong, slightly sickly smell with a hint of tomcat. Ship rats smell acrid, like wet woodash, and brown rats have a mild, quite pleasant smell rather like a digestive biscuit.