It's quite difficult to tell them apart - rooks are bigger than crows, and have lighter feathers around their face, and a paler beak - crows are usually completely black.
The easiest way to tell them apart is to think about how many of them you can see. Rooks live together in large groups. In the countryside you can often see their big nests in the tops of trees, and as the sun goes down the rooks fly around their Rookery making loads of noise. Some rookeries might be centuries old, like ancient villages.
Unlike rooks, crows are often seen on their own or in pairs.