Physician here. As others have said, Usually through scans and different tools but visually, youre right, its often a "lump" the architecture of tumours are almost always off from normal tissue. Its just rapidly dividing cells going any which way so more often its just a lump. There is often tumour spreading away from that lump too which is harder to see so they cant just go by that of course.

A common skin cancer is melanoma, and those are made specifically from the pigment producing cells in the skin, so they would be hyper pigmented and often dark. Squamous cell and basal cell skin cancers can be any colour really.