Swapping for an OD 904?

Physically, the outside appearance is the same, so it'll be almost impossible to tell just by that. You can easily tell if its lock up or not.

But to discern if its an A998 or A999 or even a A904T (904T's are wide ratio but will be weaker than the 998/999's)
You will have to pull apart the Trans and look at the sun gears.
The serial numbers on the side where the transmission pan mount can get you in the ball park. Then there is the 10,000 day calendar coding you can try and decode.
If it was made around 1980's, there is a fairly good chance it'll be a wide ratio setup.

Been through this myself a little while ago:

http://www.forabodiesonly.com/mopar/showthread.php?t=218722

Hope this helps