Download as Gizmo
http://www.nukepedia.com/gizmos/image/andrewsrotocheck
(You need to have an account with Nukepedia to download any Gizmo their Site)

Download as Group
https://github.com/AndrewComp/AndrewsRotoCheck/blob/master/Group
(Just Copy all the text and paste it into Nuke’s Node Graph)

If you are looking for a gizmo or group to check your mattes, look no further than the “AndrewsRotoCheck v1.3”.

He has added 10 unique ways in which you can see your mattes. He added the ability to invert your alpha, composite over a background plate, and my favorite feature; show/hide rough splines shapes which can help you detect holes in your mattes.

And the best part is, it’s free!