differential fluid quality usually deteriorates due to some residue produce from the meshing of your bevel gears. Usually the fluid turns darker in span of time. If you are into racing I guess you have to check it from time to time. but if your not, i think you just have to check whether there are some leakages in your differential and have it filled. Without fluid would worn out your gear easily.
For the shock fluid, I havent replace yet my shock fluid for almost a year. But I just check it regularly for sign of leakages. . .
Hope it helps. . .Cheers!!!
Xray Nt1 l Xray T3'12 l F104PRO