I have what looks like a fungal rash on my butt. How can I identify it / treat it?

Fungal rash? Most certain ways to id fungus are to have doc take a scraping, add potassium hydroxide (koh), & look at specimen under microscope for fungal elements or have doc take specimen for culture (takes about a week for most to grow). Or you can try empiric therapy: use otc antifungal like Lamisil (terbinafine) 2-3 times a day for at least a week. If rash still there or getting worse, see doc.