TF 999/998 sporting internal diff from the 904 ?

First question. Does anyone know what the first year was that the 999//998 was used in production vehicles ?
Since there does not seam to be any tell tale signs of which trans you are looking at 904/998/999 externally.
How can you tell if you've got a 999/998 and not at 904 internally ?
I know the 999/998 have lower gearset. But if you do not know how to spot the 999/998, tell me what to look for ?