If you have acne, opting to seek professional help can be a tough judgement call. After all, there are countless brilliant resources online (ahem) where you can get the lowdown on powerful DIY or topical treatments.

But what about when your questions go beyond the typical “Will these breakouts ever go away?” When you find a mole in an unexpected color—or a tiny, mysterious bump (“What is that?”)—it may be time to see an MD.

“There are three main types of skin cancer: basal cell carcinoma, squamous cell carcinoma, and malignant melanoma,” says Arielle Kauvar, MD, a New York City-based dermatologist and founding director of New York Laser and Skin Care. “Each type has different appearances, but they have early warning signs you can watch out for.”

Of course, you may not be checking every square inch of your body on a daily basis (which is fine)—but Dr. Kauvar says the American Academy of Dermatology recommends that you do self-exams at least once a month. “This is especially so if you have a lot of moles or freckles, or if you have a family history of skin issues,” she says.

The majority of the time, these conditions stem from sun exposure. “Over 95 percent of basal and squamous cell melanoma are caused by the sun,” she says. “Melanoma, on the other hand, has a 20 percent risk for areas that don’t ever see the sun—it can be inherited.”