There is two ways you can do this:
The first is using a collimator which is an artificial image set to infinity this is how the pros adjust lenses in the shop. [http://www.edmundoptics.com/technical-resources-center/videos/?ytID=ac-vVNbQt2I|Edmund Optics Vid on collimation]
The other is on a clear night with a visible moon use it as your target. Adjust the lens so it is set for the most zoomed in and set manually for infinity. Adjust the lens group so the image is sharp that should do it.
Here's one other method using a second camera [http://feuerbacher.net/photo/repair/InfinityFocus/InfinityFocus.html|Using a second SLR camera as a collimator]