Looking at the passenger side of the case, is there a cast in line that looks like a wave on the ocean, or a cam lobe? It will start just in front of the filler tube boot, and go between the cooler line fittings, and then back down behind the 2-4 servo bore. Here is a pic;
If that line is absent from your case, then you most likely have an early, non-desirable, transmission.
Have you taken the pan down to see if there is an auxillary valve body? If Aux. valve body, and/or has the casting line cast into the case on the passenger side, it is a '91.
If it is NON-AUX, and has no casting line in the case, you probably have one that was manufactured in '81. If it has the original pan, some of the early 700's actually have a very thin bottom pan, with a large "X" stamped into the pan.. Also, if the pan bolts resemble the old TH-350's, it is most likely an early trans. If the pan bolts have a washer on them, it is most likely a late-but this is not defining as they can be easily changed.
Also look for the last 3 casting numbers on the pump-732 is 10 vane and usually denotes a late unit. Anything with casting numbers 30X is an early 7 vane pump and came in the 82-84, and some early 85's.
You can also count the splines on the input drum. If it has 27 splines, it is an early input drum, and most likely an early transmission.
In regards to whether or not it is, or has been, rebuilt, usually you have to open it up and look for signs such as after-market gaskets, or other visual clues. If it is an early trans, don't waste time rebuilding it. Buy a later (86 & up) trans from a salvage yard, and rebuild that one. The cost to bring an early trans up to late specifications is prohibitive. If you have an early trans, and you do not plan on hotrodding it, or racing it, then you can rebuild the trans and it will work just fine.