Bit late replying but if you mean 'How do I see an Iray preview in the viewport window', i.e. without starting a render then there is an option for it. Look at the top right corner of the viewport (not the whole screen) and you'll see the grey bar that allows you to select cameras and various other views. Just to the left of this there should be a symbol (probably a circle/sphere) with a small arrow underneath. Click that and a list of options pops up with 'Nvidia Iray' at the bottom. Select that and you are a go. Bear in mind that you will need some kind of light source to see anything and it takes a reasonable beefy machine to keep it on full time. It can get bogged down and appear to be doing nothing sometimes, so use it as you see fit. If you do find it a bit much I recommend what the other poster said about setting up a small aux viewport and setting that to Iray as I described, leaving your main viewport at default or whatever.