You can preview a shading network, or a part of a shading network, in the Render View by connecting it to a Render Target node. Use IPR for mental ray to preview and tweak your shading graph real time.

Example

1. In the Hypershade Create bar, select Maya > Utilities > Multiply Divide.
2. Connect a Ramp (Maya > 2D Textures > Ramp) to Input 1 of Multiply Divide.
3. Connect a Fractal (Maya > 2D Textures > Fractal) to Input 2 of Multiply Divide.
4. Create a Render Target node from Maya > Rendering > Render Target.
5. + the Multiply Divide node to the Render Target node to connect them.
6. In the Render View window, select mental ray as your renderer, then select Render Target > renderTarget1 and render.

You can preview the result of the shading graph in the Render View window. Select IPR to tweak your shading graph and visualize the changes real time.