If you notice dark patches on the fence after it gets wet, then chances are that the water is getting absorbed.
This will rot your fence. Also, notice if there are any cracks in the fence. The water might have seeped through such cracks. So, dark patches show that it’s time for staining your fence.
Another reason for water getting absorbed is if the stain or paint has worn off. If you think that the original color looks faded, then that’s also an indication to put on a new stain coat.