You are correct, Overprint Preview > Automatic automatically detects if there is content in the document that requires overprint preview to display it accurately. Changes to the preference should be immediately visible, without having to quit and relaunch. Are you getting different behavior? If so, what OS are you on?
There is one other way to toggle off overprint preview temporarily when it is enabled by the preferences. WIth the Output Preview dialog open, there is a checkbox to turn on/off Simulate Overprinting. This on/off toggling only works while the Output Preview dialog is open.
Color Warnings view in Output Preview should still highlight the objects that are set to overprint. Additionally, you can use Preflight to determine if there is overprinting content in the file. There currently is no message bar or overprint indicator icon.
Also note that, even with the Never setting selected, overprint preview is automatically enabled when the Output Preview dialog is open.
I hope this cleared up your questions?