How do I know if I have a broken finger or just a sprain?

X-rays. Pain, bruising, swelling and difficulty moving the finger are all signs of injury, but only an x-ray can prove the finger is broken.
X-ray. The only way to know for sure is to get an x-ray. A finger with a small break or a breaks that isn't too badly out of place may not look too bad from the outside. Also, it's an "old wive's tale" that it's not broken if you can move it. Hope you feel better soon!