Can an ultrasound determine if a lump is cancerous? Can a dr look at the results of a breast ultrasound and tell whether a lump is benign or malignant? Or can only a biopsy really reveal this diagnosis?

Tissue. An ultrasound can strongly suggest, but only a biopsy will give you the answer for sure.
High suspicion. Ultrasound is excellent as a companion to mammography in detecting breast lumps or masses. There are several factors in ultrasound images that can accurately predict the chance of a mass being cancer, but there are limitations. Ultimately, ultrasound guided biopsy will allow a pathologist to make the definitive diagnosis on tissue.
Sometimes . Sometimes a breast ultrasound can determine whether a lump is definitely benign. If the lesion is a cyst (an encapsulated fluid collection) then it is definitely benign. Some lumps are normal lymph nodes; these have a very unique appearance on ultrasound and can be considered benign. Otherwise, a solid mass can have an appearance which is suggestive of a benign lesion such as a fibroadenoma; or they can appear suspicious for malignancy. These solid lesions are often biopsied in order to get a definitive, tissue-based, diagnosis from the pathologist.