The most obvious sign is the condition of the fluid. If its getting dirty and smells burnt fast - its time to start saving for a tranny rebuild. Other signs are delayed engagment when shifting gears, both when the transmission is shifting through its gear range when accelerating, or when you are actually changing gears with the selector. As the tranny gets hotter when driving, especially in stop and go traffic, if you notice any long shifts, usually a sign of a tired tranny. You have to remember that the shifting is controlled by the truck's powertrain computer, which, in turn takes many variables into consideration before a shift takes place(adaptive shifting). So shift timing, shift length can be different at different times.
The thing with trannies is unless there is a serious or obvious problem with the tranny, the tranmission can sometimes go for a long time without actually failing, or it could drop out on you the next time you pull out of your driveway.
When I did my tranny I used the sonnax sure cure kit,
http://www.thesurecure.com/parts/parts/4L60-E(in).pdf, upgraded some internal parts to make the tranny more stronger. when it was done, it shifting quicker, less overlap, but smooth. Hope this helps.
