Since the City of Detroit return is electronically filed with your Michigan State return, once you receive an acceptance from MI, you can consider your Detroit city return efiled as well. You can confirm that the City of Detroit has received your return by calling 517-636-5829 from 8a-445pm Mon-Fri or by emailing Treas_CityTax@michigan.gov.
http://www.michigan.gov/taxes/0,4676,7-238-73294_73296_73297---,00.html