There are a few options for going back and forth between magnifications of an area.
(This is my preferred method) If you're working on multiple screens, you can have 2 windows open of the same image. Go to Window > Arrange > New Window for [filename]. Then have one window zoomed in on where you're working and the other at 100%.
Hold down Z to temporarily engage the Zoom tool and drag to zoom in or out.
Use Ctrl/Command and - or + to zoom in or out. (You can also use Ctrl/Command + 0 to Fit to Window or Ctrl/Command to view the document at 100%. You can also assign your own keyboard shortcut for zooming in at 200% which is under the View menu).
Open the Navigator panel (Window > Navigator) and use the slider to zoom in or out and drag the red box to position the zoomed-in area. You can hold down Ctrl/Command to use the zoom tool over the thumbnail to also zoom into a particular area.