With low tbi and pressures in left toe and abnormal waveforms in both toes but all normal in both ankles, how can you tell whether it's frostbite, vasculitis or systemic disease such as scleroderma?

Not possible: A thorough history, exam and further imaging with an angiogram would be needed to differentiate.

Multiple: Most skin conditions appear visibly different. If there are two conditions which look alike, the patient's history and symptoms usually point to one diagnosis. Lastly, if the history does not clarify the diagnosis, dermatologists do biopsies of the skin. This is an objective measure whereby the skin cells are visualized under a microscope and usually definitively define the diagnosis.