Isn't there a tag on one of the cover bolts that has the gear ratio on it? (If it hasn't been removed.)
You can tell the ratio by spinning the yoke. If both wheels spin the same direction, it's a posi. In this case, count the revolutions the yoke takes to spin the axles once. If you turn the yoke and the wheels spin opposite directions, it's an open diff. You'll have to hold one wheel still and count the turns the other takes.
At least, that's how I've done it in the past.