I once spent money to have some Amsoil synthetic ATF analyzed after 50k miles of service in my sister's minivan. This was probably one of the first vehicles I had used it in, and it was certainly an application that got lots of abuse and hard service, although it did have a decent size factory aux. ATF cooler installed. The fluid did not look bright red anymore, in fact it did not have much red left in it. Also, it did not smell new and was starting to get an oxidized smell to it. I do not have the numbers from that analysis anymore, but they said it was fine for continued use. We had already changed it again, flushing the whole unit out to get new fluid throughout. It has been changed once more since then, and this van currently has well over 300k miles on the original transmission.
But, getting back to your questions, I look at the color and smell of the fluid to get an idea of how it is doing. The smell tells me more since I seem to be able to smell oxidized ATF a mile away. Comes from the days in tech school rebuilding burnt GM TH350's.
ATF holds up much better usually when there is adequate or even upgraded ATF cooling. Consistently higher running temps degrade fluid faster. Higher quality fluids resist this abuse better. Most factory-installed ATF fluids are much higher quality than fluids from 10 years ago.
Another thing to consider about ATF is the mostly inadequate filtration from the factory design. It would help to have some type of filter in one of the cooler lines to help always keep the fluid clear of smaller wear particles.
