Sarcoma a skin cancer? how diagnosed?

Can be. Biopsy: Sarcoma is a cancer of the connective tissue, muscles and bone. Sarcoma, e.g., fibrosarcoma, can develop in skin. Diagnosis is by biopsy and examination of the excised tissue by a pathologist.

Sometimes: Sarcomas cover a large group of tumors that arise from soft tissues (skeletal muscle, smooth muscle, bone, cartilage, blood vessels, nerves, fat, connective tissue). Just below the skin, all of these structures can become malignant and give rise to a sarcoma. A biopsy with microscopic examination (by a pathologist) is required, with special studies usually required to confirm the diagnosis.

Not skin cancer: Sarcomas are tumors which arise from deeper tissues under the skin. They can come from different types of cells, and are named to reflect where they came from. Diagnosis is generally made by some time of biopsy and treatment depends on the type of tumor, where it is located, any local or distant spread, etc.