hi friend
i am not sure how well you could do structural differentiation in shoulder especially in impingement.
what the recent studies suggests it as "subacromial impingement syndrome"
you can do a cluster of tests and clinical signs .....
pain in subacrmial area with palpation
pain ful arc sign
full can test
neer impingement test
speed test
hawkins kennedy test

also we can approach it based on movement impairment syndrome concept.analyse the movements and functional activities
keeping in mind....post partum related changes