You probably never thought of seeing a podiatrist for a skin check, right? Although malignant melanomas are not common in the foot, they do occur and I have seen them.  Here are some examples below.

You don’t. The only one who can tell is a pathologist who looks at the lesion under a microscope. Sure, you can use diagnostic factors and guidelines to give a “probability” of it being malignant, but the only true way to tell is to look at it from a cellular level.