I recently ran across these photos from last August when ButterCupp calved. I realized that they are a good photo example of what to look for when trying to determine if calving is imminent.
You can very distinctly see the udder and how full and tight it is. In addition, notice how "shiny" the teats are. The shiny teats are a good indication that calving will be soon. You can also see the prominent pin bones and the loose tail head. These are all a good indication that a calf is on it's way and soon!
For additional information, check out this informative article from Oklahoma State University.
ButterCupp gave birth within an hour of my taking these photos.