If there is a question about diagnosis of warts, and there is a possibility you could be dealing with a callous or corn, it is important to shave the skin and look for the following:
1) interruption of skin lines by a swirl pattern,
2) black dots which are actually thrombosed capillaries in the upper portion of the dermis. Generally, both findings are present, but either one would help to ensure diagnosis of warts. Calluses have continuous skin lines running through them, and corns have a central keratin plug without multiple circles or swirls interrupting the skin lines.