Look at the color too. Fresh automatic transmission fluid (ATF) is bright red. Almost pink. As it ages, little bits of metal are worn off your transmission and the ATF turns more brown, and eventually black. If the ATF dipstick reveals something that looks like used motor oil, you're in need of a full flush/change.
And the smell is near impossible to wipe from your skin. The only thing worse is diff fluid. Just thought I'd add that in to help justify the cost of garage change.
Good point about the flush/change. If it's already black, it needs a double run through of fluid. Empty, fill with new, let it churn a bit, drain again to rinse out the crap and then fill with clean again. I mention it just so it's not a surprise when you see it on the bill or if you decide to do it yourself.
On most vehicles, its a pretty easy job. just depends where they put everything and how easy it is to get under your car.
