How do you know if you have a fungal skin infection or eczema?

A dermatologist...: ...Will take a sample of the scale from the rash, put it under the microscope, and look for fungal elements (we call them "hyphae"). If they are present, it's a fungal infection; if not, eczema or something closely related in most cases.

Dermatology: A dermatologist can evaluate and treat according to diagnosis. Cultures can be done of the skin to help with diagnosis. Topical and oral treatment can help. I recommend not letting it go to long without treatment.