Thrush that doesn't resolve could be a sign of diabetes. But if someone is dealing with a diabetic rash, they must have consistently very high blood sugar levels. And this will cause other symptoms in addition to the rash like excessive thirst and urination, nausea and an odd taste in the mouth.
The only way to know if a rash is being caused by high sugar levels is to get a glucose tolerance test.
I never got a diabetic rash before my diagnosis but I had all other symptoms of diabetes. I got my first diabetic rash fifteen years after I was diagnosed. My tablet medication was no longer enough at that time and I couldn't keep my blood sugar in control. I now use both insulin and tablet medication. Thankfully, I haven't had any rashes for a while. Diabetic rashes can be a nightmare.