You can flatten (see bottom of Layer menu) but you don't have to. There's a Stamp Visible command that will create a new layer containing a composite of visible layers. The new layer will appear above the highest targeted layer or on top of stack if no layer is targeted. The new layer can then be inverted.
Mac = Opt+Shift+Cmd+E then Cmd+i
Windows = Alt+Shift+Ctrl+E then Ctrl+i
...now that was slick.
Thank you Conroy. This is why the adobe Forum is the best forum.
Did you not mean to mark Conroy's Reply as the "Correct" answer?
Thank you. Adobe has a team, that is tasked with supporting the forums, and they use things, like "Correct" answers, and helpful answers, to go to upper-management, to campaign for more support of these forums. The little "Was this helpful Yes No?" are also being monitored, as well.