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This varies by the mail client that you are using, though every mail client will do it similarly. In general, they will all put some indicator in or around the header of the mail message to indicate that the message has been digitally signed and that the signature is valid or invalid.
Various mail clients:
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Currently there is no way to verify a digital signature in Google Mail. Please use Webmail or real mail client to verify the authenticity of the digitally signed messages. We are investigating options with Gmail.
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