Use a white or light colored water dish - any drowned mites will look like pepper in the bottom of the dish.
Occasionally mine will soak even with the humidity at 70% or more, starting a day or so before signs like a pink belly.
Also, if your hides are too big or too open BP's may try to use the water dish as a hide - they like very dark hides that are just slightly larger than they are, with one entry. The half-log hides that most people get from pet shops are inappropriate for BP's unless you block off the entrances with plants or substrate so there's just a small access hole.