Hello and welcome to justanswer.com,
let me see if I can answer all your questions.
first, there is no susch thing as a car being too old to change the transmission fluid. The fluid should be changed every 30,000 miles regardless of the age of the car.
There is a dipstick to check the fluid level. it is located in the front, top of the transmission. I can try and send you a picture of the location. the fluid is checked with the vehicle on a level surface with the engine off. the fluid should be a reddish to reddish brown color and not have a burnt smell to it.
It is not true that changing the fluid on old cars does more harm than good. the only time it could possibly an issue is if it has been a long time since it has been changed. this may cause some issues but chances are the transmission was faulty anyway.
bad or very weak motors mounts can cause a jerking or banging when shifting into gears. if after checking the mounts and replacing if necessary and changing the fluid you still have issues , only then should you condemn the transmission,
believe me there is a dipstick here is a picture of the location
if you are having a transmission related issue I would hope they would at least drive the vehicle first before giving you a diagnosis.