Here's the easiest way to tell.
Look at the bottom of the pistol frame at the magazine well. If the surface appears smooth (left), the frame is probably forged. Since this was written, I have learned that some early cast frames did not have the ripples. This IS apparently the case with an "NX" ('93) Mk III I have seen pictures of It does not have the grooves shown above. If it has the longitudenal grooves or "ripples" (right), it is cast.
Purists will opt for the forged frame every time...and in my view, there's nothing wrong with that.
If my intention was to shoot thousands of hot-loaded 9mm rounds, I'd personally go with the cast frame. I do believe that they handle the warmish ammunition a little better in the long run. With standard pressure, it doesn't make me any difference.
PS: An updated article on this is here: