First we need to open multiple scenes at once. Open the first scene you want to examine and then right click on the second scene in the Project Window and select Open Scene Additive. Now you have both scenes opened at once and you can even drag and drop GameObjects from one scene to another in the Hierarchy Window. However, they will be overlapping, so it won't be pleasant experience. You can temporarily move all GameObjects in one scene to some other place (preferably for the scene opened in the additive mode, since you will probably want to discard changes in that one) (e.g., select all Game Objects in the Hierarchy Window and then Create Empty Parent, then translate this parent so the scenes are apart enough) and then open another Scene Window and navigate in this scene window to the second scene - now they will be visible side by side.