“Bad fluid”, as in…“did I put the wrong type of fluid into the trans”?
"Bad fluid", as in…“is the fluid overdue for changing”?
The answer is “yes”, to both possibilities, but only you can answer the question completely by checking both your service records and your dipstick.
If the fluid is any color other than a pinkish red, and if it has a burnt odor, then you have bad fluid.
If the fluid wasn’t changed during the past 3 years, then you most likely have bad fluid.
One of the problems with buying low mileage used cars is that the previous owner may have ignored the elapsed time factor when it comes to maintenance. On the basis of elapsed time, this car should have had the trans fluid (and filter) changed 4 or 5 times in its 15 year life. While you can’t turn back the hands of the clock to undo lax maintenance, you can (hopefully) extend the life of this transmission by doing a fluid & filter change right away, using the correct specification fluid.
Also…instead of “sitting for years”, this car may have been driven less than 3k miles per year by the previous owner, and that can be…ominous…if that owner was not very diligent about oil changes. That type of usage is usually a situation of an owner who does only short-trip, local driving, and that is very bad for an engine–especially if the oil isn’t changed every 6 months. Unless you can confirm that the car got oil changes every 6 months, I would suggest an immediate oil change, followed by two more at one month intervals in order to try to clean out accumulated sludge from that engine.