Yes, mange mites are microscopic. Inexperienced vets will often try to do "skin scrapes" to verify that mites are there. I personally would not allow a vet to do this, it is painful to your guinea pig and false negatives are common. It's advised to do treatment anyway if there is any type of itching because the treatments are not dangerous to healthy animals.
We are as gods to the beasts of the fields. We order the time o' their birth and the time o' their death. Between times, we ha' a duty. - Terry Pratchett.
"Men have forgotten this truth", said the fox, "But you must not forget it. You become responsible, forever, for what you have tamed. - Antoine de Saint-Exupéry