How can you know if you've sprained your finger?

Pain: A sprained finger is painful & swollen. It is well alligned and it is possible to maove it with difficulty. However an x-ray should be done to rule out fracture.

History: What was the mechanism of injury? Was the finger "jammed, " twisted, etc. Is there swelling or bruising? I generally recommend a very cautious approach with fingers, suggesting x-rays unless a very minor injury. Missed fractures or mistreatment of a sprain can have very negative and permanent consequences with finger injuries.