I'm sort of late to the party but one way that has helped me is to look at the XML definition of the thing you are trying to query.
In the snipped I'm sharing I was trying to get to the actual argument string (TestStand Expression) passed in the VI call parameter for the input "My Numeric" (I passed in "Locals.MyLocal")
As you can see, the name of the input along with the TestStand Expression passed in is displayed in the XML of the step. I used that to adjust my query.
Hope it helps someone.