All strains are a little different, but Sativa's have a longer season than Indica plants, and I think that they don't show the signs like an Indica plant, as true Sativa doesn't typically have the big coating of Trichomes that you find on a lot of Indica. Hash plants, which are generally pure Indica strains, have tons of Trichomes that can be pressed for hash, while Sativa doesn't seem to have as many. It gets a lot, but they are different and not nearly as many, it seems. I have compared Sativa plants to trees during fall, and the comparison is pretty similar. Sativas will start to show yellowing leaves starting at the bottom of the plant and working it's way up. While this is going on, the smaller fan leaves that grow on the buds will start to change color, often showing purple if the temps drop during late flowering. Like trees, the leaves lose their chlorophyll and show their colors that were hidden by the chlorophyll. The buds are hard to time, since they often look ready after about 8-9 weeks, but they just keep going, getting thicker/heavier and getting greasy feeling. The plant might appear to stop growing, but the buds will keep getting fatter. When most/all of the large fan leaves are dropped and the smaller leaves are changing colors, that is when I take them down. I personally prefer to let them go too long than not long enough, as the flavor really develops at the very end. I've have sometimes left a Sativa just dry in the bucket, since they are really spent after flowering, and sometimes just die regardless of what you do with them. By just leaving them in the bucket for a few months after the lights had been switched yields a really unusual and hash-y tasting bud. Often they will be bright gold due to the chlorophyll being totally consumed from the plant tissues drying so slowly.
Sorry for the long ramble. I just love growing Sativas and not many folks nowadays grow them because they don't give the big hard nuggets that everyone craves or have the smell of a candy cane or similar stuff. I guess that is what I grew up with (Mexican/Vietnamese/Panama/Colombian) and I've grown it for decades. It is different to grow than Indica, with both having their own unique issues. I enjoy it for the sake of doing it and after a while you just kind of look at a plant and know what it needs or doesn't.