Cut outs - 2 and 1/2 questions

There really wasn't anything on cutouts in the memories so I'm hoping this hasn't been answered in some previous post that escaped my search. Is there anyway to preview cutouts on a dark colored background short of putting a layer behind everything and filling it with black or a dark color? Because I noticed that even when I do that, I still miss some spots where there's still some part of the image left over that I didn't see and didn't erase, and I only become aware of it when I post the icon in the icon box setup. By then though some people may have taken my icon before I realize it and can fix it. And when it comes to outlining cutouts, I use the Stroke option but the stroke sometimes comes out pixelly rather than smooth looking. Is there a way to fix that? Or is it just because I saved a transparent icon as a png versus a gif?