with an automatic you wont be able to see too much visually unfortunatly. Its worn fluid channels and clutch material that your dealing with, nothing thats too easy to spot without taking the tranny apart first. The only thing you can hope to "see" is whether the fluid is burnt up (black and smells like death)... Or if the bands need adjusting. There are big bolt looking things on the bottom of the tranny you can see after pulling the pan and you turn them to adjust your bands... A chilton or haynes manual will tell you step by step how to adjust them.
But 300 bucks is a pretty sweet deal for an automatic that is in working order, if you've got the money, 300.00 isn't a bad insurance price.