About the only way you can tell at first is to give it 'the sniff' test. If the oil is dark brown or if it is empty smells burned, walk away. Then the sight test - and that's to pull the pan and look at the filter and the sediment as Roger noted. I wouldn't put much money into a used transmission when there are several fairly good companies who will sell you one for less than a $1000 bucks and set up for a low level of performance. Automatic Transmissions - SummitRacing.com. The first 3-4 here are $660-950. I'm not going to get into the merits of any of the rebuilders, but all will do the job just fine with an occasional dud. My TCI Ford C4 Street Fighter has been fine.