Q: What can you do to see how healthy a transmission is / how long it'll last? How much do these tests tell you?

What're the best tests you can do to gauge how healthy a transmission is / how long it'll last, aside from literally taking it out of the car and disassembling it?

For example, you can check the fluid to see how cloudy / dark it is. You can drive it to see how well it shifts / if it lags.

What is the suite of tests like this you can do to gauge how how healthy the transmission is / how long it'll last?

Also, how accurate of an indicator are these tests? Knowing the results, how confident can you be that the transmission will ultimately last a long time?

My car has 161000 miles.
My car has an automatic transmission.