The best way to know for sure is honestly just to get it checked by a derm or your doctor - they can do a quick scraping or look under a microscope if needed. But there are some things you can pay attention to before you make that appointment. Fungal infections tend to have that really sharp, defined border that doesn't blur into the surrounding skin, and they often feel kind of scaly or almost leathery. Eczema is usually more of a gradual fade into normal skin, tends to be drier and itchier, and often shows up in clusters or patches that spread irregularly.
I dealt with this exact thing on my shoulder a couple years back. I was convinced it was my usual eczema flare because I get that in winter, but three weeks of my normal routine - ceramide moisturizer, avoiding irritants - didn't touch it. The thing that made me realize it wasn't my usual eczema was that it was almost weeping a little bit and had this kind of darker, more defined ring around it. Went to my doctor, turned out to be ringworm (which is ironic because I spend so much time at the gym), and the antifungal cream cleared it up in like two weeks. So yeah, if something's not responding to your normal eczema routine after a few weeks, that's your sign to get it looked at.
One thing worth trying in the meantime: keep the area completely dry and don't cover it up with thick creams. If it's fungal, moisture can actually make it spread. If you're dealing with eczema, you'd normally want to moisturize more, so that difference can actually help you figure out what's going on before you see someone. But don't self-treat with antifungal cream if you're not sure - that won't help eczema and you might just delay getting actual treatment.