Use this command, (go to java < jdk < bin path in cmd prompt)
$ jarsigner -verify -verbose -certs my_application.apk
If you see "CN=Android Debug", this means the .apk was signed with the debug key generated by the Android SDK (means it is unsigned), otherwise you will find something for CN. For more details see: http://developer.android.com/guide/publishing/app-signing.html