Grooming Do my Guinea pigs have mites?

If you could post a picture, you'll likely get a better educated answer. It could just be a fungal infection, or nothing at all.

There is no way to know for sure if your pigs have mites. A vet visit would cost you money and likely result in the vet wanting to do a skin scraping, which is painful for the pig and misses the mites half the time anyway.

Where is the bald spot?

Ditto @Guinea Pig Papa.