Listening for श vs. ष
I can match श and ष to the correct transliterations, and I think I have studied enough (through non-Duolingo resources) to understand how they are produced differently. I have looked up other audio examples of IPA /ʂ/ (श, śa), and it does sound different than /ʃ/ (ष, ṣa) to me. I have also been working hard on saying them correctly myself.
. . . yet I have a very difficult time telling them apart by ear in the Duolingo examples! When practicing the lessons letters, I can play examples of both back to back, and each one sounds exactly the same. Does anyone have tips?
same...can't hear any difference, but it is easy to hear the difference when you hear a real speaker: https://www.youtube.com/watch?v=bxyDtRLd1I4
There were a couple others that I thought were also identical, but I haven't come across them in a while...it was a pair with the "u" sound
 I found the other pair I was referring to above: चु and चू
There is no pronunciation difference between the two letters. Earlier, they used to be pronounced differently but the modern pronunciation has converged to श so both ष and श are pronounced like श.
In the video the speaker is trying to forcefully pronounce them differently, naturally people wouldn't do that, only language purists would. You can see the like dislike ratio to get an idea of that.