This depends on the e-mail service and app you're using. You should check your 'sync' settings for the account if it's not a GMail one and if you're not using the native GMail app. On most stock and 3rd party apps there should be a menu button that allows you to see other folders (in this case, the SENT) one, but if your sync settings aren't set to synchronize the Sent folder, you might not be able to see any messages there.