I've got a local guy that is selling one with a trans brake. He claims that it was in working order when he removed it. What can I look for to ensure I don't get shafted?

I know to check the fluid to make sure its not burnt or have metal shavings. But if it has been drained, what can I look for then?

 

Theres not much really you can tell from the outside. Make sure it has a t-brake solenoid on the side by the filler tube. Hopefully its a short tail, which the tailhousing is about 3 inches long. 400's are some pretty stout trannies, you should be ok.