There are several indicators (it differs for each of the 3 main types of skin cancers) but generally, the following features are considered high risk: 1) Fair skin, than burns easily 2) Large number of moles 3) If you've had a previous skin cancer 4) A history of sun exposure in your job or recreational activity 5) A history of chemical exposure 6) A family history of skin cancer
If you are at high risk, it is usually recommended that you have at least a yearly skin check with a doctor who is qualified in diagnosing and treating skin cancers.