Any good way to tell if an injury (to the wrist) is a fracture or sprain?

An x-ray is the best choice for diagnosis. But if you are wanting to to diagnose your self, here it is. Most generally, a sprain to the wrist, swelling will last for a few days due to the stress on muscles, cartilage, and ligaments. While a fracture, hair line, or in-line, swelling may reduce in 24 hours. These kind of fractures can go undetected with an x-ray. Off set and compound fractures are very painful as you could guess and need medical attention.

An x-ray.

X-RAY

A xray is the best way. But in generality, a sprain is more painful than a fracture.

Again, an xray is best. I cracked my tibia a little over a year ago and just thought it was a bad bruise. I didnt goto the doctor and it healed a little crooked. I didn't know this for a year. I didn't have anything done because it wasn't bothering me.

Feel it first, gently. If there's obviously something wrong, like a bone in the wrong place, it's probably a fracture. If you can rotate your wrist in certain directions but not in others, it might be a fracture. Otherwise I'd say it's a sprain. Either way, put lots of ice on it.

does it move at all?

I had a sprain in my wrist once, but it ended out to be a hairline fracture! Just goes to show, you should go to a doctor just to be safe.

SOMETIMES DEPENDING ON HOW YOU PULLED THE MUSCLES,A SPRAIN CAN BE MORE PAINFUL THAN A BREAK,AS ALL YOUR LIGAMENTS WRIP TOO.GO TO A/E AND GET AN X-RAY IF YOUR REALLY CONCERNED.HOLD YOUR WRIST OUT IN FRONT OF YOU WITH YOUR HAND UPWARDS AND STRIGHT,NOW SEE IF YOU CANLIFT YOUR WRIST UP AND DOWN,OR FROM SIDE TO SIDE,LISTEN AND FEEL THE BONE,YOU SHOULD GET A GOOD IDEA.IF ITS A BREAK NO MOMEMENT WILL BE POSSIBLE,A MUSCLE WRIP WILL HURT,BUT USUALLY NOT BROKEN IF MOMEMENT IS POSSIBLE.WRAP A BANDAGE TIGHT ROUND IT,TRY TO KEEP AREA AT REST.

I do not know. Go to the doc tell him you think u broke it or sprained it or fractured it. Then he will ask if this hurts while poking you and answer truthfully. Then he will diagnose u and may put u in a splint or a cast.