Only prerequisite is that you have installed the Internet Explorer Developer Toolbar
Go to your feature page "Site Settings > Site Features" or more directly go to /_layouts/ManageFeatures.aspx?Scope=Site
Here you will find the list of deployed features.
Now choose "Select Element by Click" en select the "Activate" of "Deactivate" button of your feature.
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Now you can see the "GUID" in the elements window
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