How could I know if my elbow is broken?

Pain. Pain, swelling, difficulty moving all would be potential signs of a fracture. I would not hesitate to be seen.
Pain, loss of motion. I agree with my colleague who has already provided an answer, but there are some symptoms that while not specific, might point you in the direction of having a fracture. Significant loss of motion after a specific trauma or ongoing, unrelentling pain might signal a fracture. You can feel the ulna bone in the forearm significant tenderness or a palpable defect may be present. Good luck!