Search for kaleidoscope apps on google. It looks like there are quite a few available. I imagine this is what the shop employee was showing you. I don't believe you will ever find an app that will really show you what a fabric will look like in a OBW, however, since it depends on how you cut the fabric and how you arrange the design. But an app can show a kaleidoscope effect, giving you an idea of how well the fabric will work in a OBW.