I'm assuming you have and image and you've applied a layer effect - Glow- to it. Duplicate the layer, rasterize the layer (Do this by creating an empty layer and merge the two. Now you can make a selection out of the image and the glow - apple click (Mac) or Control click (PC) on the layer Icon. Now create another layer and fill the selection, you have a glow by itself.
Another way to do this if your working with a very dark or black image is to apply a "Screen" transfer mode to the layer (Layer pallette, top left of the pallette, pull down menu with "Normal" in it. Often Times I will take my image, fill it with black, add a glow or some other filter and use a transfer mode to loose the image and keep the effect. Try some of the other transfer modes. You can create some awesome effects