If the part is clear, crossed polars can be used to examine the
orientation of a polymer part. The degree of orientation can be
quantified (Senarmont or Babinet compensation) and this could then be
related to a stress.
I can go into a lot more detail, but before I do so I would like to
know if this what you are interested in. I don't want to waste time
for either of us if this is going in the wrong direction.