“How can you tell the difference between a malignant and benign tumor?”

Microscopic analysis by qualified pathologist.

Starting that benign tumor don’t spread or invade tissue. Malignant cells metastasize to other tissue and organs. Benign tumor has relatively well differentiated cell. They are often surrounded by an outer surface (fibrous sheet of connective tissue) and stay contained within the epithelium (the outer envelope of the cell. Benign cells have a normal nuclear / cytoplasmic ratio, and the nucleus is round with smooth nuclear membrane, and nuclear density. The malignant cells the nuclear cytoplasmic ratio is decreased and dense, and the nuclear membrane is irregular. The nucleus of a malignant cell is large in comparison the normal cell. Thank you.

Benign is usually defined without calcification, but malignant tumors are ill defined and often with calcifications. A biopsy is the gold standard.

That varies a lot depending on where the tumor is (skin, abdomen, brain, etc.) Certain tumors, like lipomas (which are benign fatty tumors), are fairly easy to diagnose just from the exam. We can get clues from the history and exam (characteristics of the tumor itself, other symptoms, family history), but those only give so much information. In certain instances lab tests and imaging provide more clues.
Generally the gold standard for diagnosing is a biopsy. With a biopsy, the pathologist can look at the cells themselves to determine if they are benign or malignant.

Depending on the location, you can distinguish by imaging, but many times, biopsy and tissue analysis is needed.