Fungus usually glows under a blacklight 80% of the time so you can cheaply check with one of those.
I don't bother checking, I use
Flowers of Sulfur in the dust, 1tsp to a Cup or 1 Cup to a 50lb bag of dust. With a healthy immune system you'll see the chin start to re-grow fur within a week or two. I get my sulfur from the ranch outlet store in town, but I linked to the type I use on Amazon so you can see what it looks like.
Alternatively, you can use Tinactin in the dust. Same ratio. It will take 4-8 weeks for the fur to grow back, but it is the same thing.
I wouldn't use the cream, the part that is bald is already done, to get the ringworm you have to pluck 1/2" around the furless spot and rub it into that skin (or grind it into the fur and make sure you thoroughly coat the skin). Easier to just use dusts.
Don't worry, it's not a big deal. It usually happens any time something stresses out the chin to the point it's immune system is compromised. That could be weather, temp changes, noise volume, changing your ritual, etc. Some chins seem to carry it and are a bit more high strung so break out more often. Some never seem to get it.