My seven year old fell on his shoulder and says it's sore, but it doesn't seem to limit his motion much. Were it fractured, wouldn't it be obvious?

Possible Fracture. If there is localized pain in the shoulder which persists, this may indicated a contusion or a possible fracture. Fractures which are nondisplaced may not cause significant pain and dont need to be obvious. He should be examined by an orthopaedic surgeon.
No. In children, sometimes a minor fracture can be less than obvious. The bone that is most likely to break is the clavicle, otherwise known as the collar bone. If he is still complaining of pain after a few days, take him to the doctor to be checked out.