It's still warm enough here for hedgehogs to be active. Droppings are probably the best guide: rats' are dark brown, smooth and cylindrical and about 2cm long. Hedgehogs' are bigger and more irregular, often almost black and sometimes containing beetle wings and other bits and pieces. Are you able to shine a torch under the shed? Hedgehogs will probably have brought in leaves to make a nest. Rats are more likely to dig a hole.