I'm not sure exactly what you mean here,

The view port window shows a preview of your scene pre-render or alternatively you can open the Aux view port and set to Iray for a real time IPR (Interactive Progressive Rendering) Iray render preview.

Generally the easiest way is to start an Iray render and then you would only need a few iterations rendered before you have a grainy view of your render which would serve as a preview.