I have the little gizmo that MTS has mentioned ... just hold it to your eye and you see several of the same block. However, my eyes have not masted the "how to" of it!!
The OBW etc mirrors ... I purchased two mirrors at the $ $tore, taped them together so I can stand them on end. Then just move them til I see 4, 6 or 8 reflections. I even keep them in my truck, ready at a moment's notice, just in case I see some potential fabric I need to check for it's OBW'ability! :)