Create a fixed relationship with the option 'use distance between faces as default offset' (don't forget to uncheck it afterwards!). The value you are looking for should be listed in the edgebar as well as in the variable table.
I think you can also do a floating relationship. After doing so, select the part that contain this relationship. The value you are looking for should be displayed as a dimension.
You can also create an assembly sketch in a plane perpendicular to the other two and place a dimension between the 2 planes using include if necessary.
