Rodent droppings are small and don’t really smell of anything, but they are conical and solid. Insect droppings are (as you would expect) a lot smaller than rodents but also contain body parts, wings and shed skin.
When it comes to spiders, they make webs to catch their prey. They know their stuff when catching pests, so if you find spider webs in your house, look for other nearby places where pests might be getting into your property.