We had the same problem, and used the most simple thing ever. :)
Just grab a protractor and measure the angle of the shock on your computer screen.
You can also probe any two points on the shock wave (every software has this tool) and using the coordinates of those two dots you can calculate the exact shock angle.
Import the file to Tecplot, and probe several points on the shock wave and the reflection wall. Computer the angle by the coordinates(tangent function).
The question is very good. Usually shock wave angle is treated as an angle of iso-line (\rho_1 + \rho_2)/2. Subscript 1 is before the shock and 2 is behind. You can export the figure in *eps or *png to measure the angle of this isoline in inkscape, for example. However it is not so clear what to do if the flow has great gradient just after the shock.