Just curious: how do you tell the difference between dermatitis herpetiformis and herpes gladiatorum?

Easy: Suppose you're sure it's one or the other, which you probably aren't. DH is symmetric. HG is wherever you got rubbed on the mat with the wrestler who had the herpes lesion. HG is likely to come up all at once, while DH less so. DH will usually test positive for anti-gluten stuff. Hope this helps. There are a few other painful small-blister diseases as well -- read up. Best wishes.