There is a lot of talk about the Psoas or the Iliopsoas muscles around the yoga community however, it is an often more difficult to understand muscle than we like to make out. We like to blame a lot of things that we cannot do on a 'tight psoas' but the reality is, our bodies don't recognize individual muscles the way we see them in anatomical books. It's much more complicated and with the fascial connections through out the body, the motor neurons being connected to several muscle fibers, the beginnings and endings of muscles being gradual it is difficult to really isolate one muscle.
In saying that, there are some interesting ways to inspect the Psoas. This video demonstrates one way of several that we explore in the teacher training.