Cheeks, just like any muscle, can bruise when injured by trauma. Not all facial trauma requires acute medical care. Most can be treated with icing the affected area and analgesia with NSAIDs such as ibuprofen or acetamenophen.
However, there are certain symptoms that require immediate medical attention:
1) Any facial asymmetry, inability to close the mouth, visible or palpable boney deformities, and extreme pain with jaw movement should warrant a facial xray to rule out any facial bone fractures.
2) Clear or pink nasal discharge is suspicious for a skull fracture and leakage of cerebral spinal fluid and should warrant a closer examination with xray and/or CT scan.
3) Any visual disturbance, facial numbness, or hearing impairment could be indicative of a brain or cranial nerve injury which prompts immediate medical attention as well as xray or CT imaging.