Landmark registration is a reasonable approach.
I find fading between the two volumes which should be registered to be the most effective way to evaluate registration quality (better than checkerboard, see discussion here for another alternative approach). You can achieve this in Slicer by selecting one volume as the background volume and the other as the foreground volume, and then use the foreground opacity slider to fade the foreground volume in and out. See User Interface — 3D Slicer documentation for the controls. If you link the slice views (the linked/unlinked rings), then the foreground and background selections and foreground opacity will all be linked across the slice views, and you can view the fading in the other slices without the slice controls getting in the way.