How to get a WPF Viewbox coefficient of scaling applied

In case we use WPF (Silverlight) Viewbox with Stretch="UniformToFill" or Stretch="Uniform" when it preserves content's native aspect ratio, how could we get knowing the current coefficient of scaling which were applied to the content?

Note: we not always know the exact initial dimensions of the content (for example it's a Grid with lots of stuff in it).