@turquoise-- Actually, most doctors do have a biopsy done. My doctor certainly did and it was not only to check for malignancy but to actually confirm that it's a hidradenoma and not something else.
I also think that most doctors know what to look for and can easily tell if a hidradenoma is showing signs of potential malignancy.
My hidradenoma was malignant and my doctor knew something was off when it kept growing and coming back after removal. But you are right that the only way to confirm is biopsy.