How do I verify that an Android apk is signed with a release keystore?

Here is the command to know the given apk is Signed or Unsigned..
(goto -> Java -> jdk -> bin path in cmd prompt)

Mac

$ jarsigner -verify

Windows

C:\Program Files\Java\jdk1.8.0_91\bin>jarsigner -verify

Output for Signed apk.

Output for Unsigned apk.