How can you tell if freckles are cancerous?

Freckles are not can. Freckles are benign collelctions of pigmented cells. If you are concerned you can show them to a dermatologist. But you need to separate freckles from moles(also called nevus or nevi). Moles are sometimes precursors to melanoma. But they have to change in size or color before you can get concerned that a mole is changing into a melanoma. Freckles do not have any bearing on developing cancer.