ctrl-click selects the layer contents, and only the layer you click on. ctrl-shift-click selects the first layer, and adds to the selection any layer contents you click later. just like doing shift-drag when marquee selecting, only with layer content.
If your paint is on more than one layer, then ctrl+click the first one to select opaque areas, as Harle said, and then ctrl+shift click the rest of the paint layers' thumbnails in layer palette to add their contents to selection. Then you can go to selection> modify> expand. If you have multiple layers selected in layers…