OS X - how to check if installer is signed
You can use pkgutil --check-signature YOUR.pkg. Sample output:

Package "Silverlight.pkg":
   Status: signed by a certificate trusted by Mac OS X
   Certificate Chain:
    1. Developer ID Installer: Microsoft Corporation
   SHA1 fingerprint: AE D0 A7 C5 31 01 2B 70 D7 FB 49 5A 23 30 3A 67 05 36 5A 11
   -----------------------------------------------------------------------------
    2. Developer ID Certification Authority
   SHA1 fingerprint: 3B 16 6C 3B 7D C4 B7 51 C9 FE 2A FA B9 13 56 41 E3 88 E1 86
   -----------------------------------------------------------------------------
    3. Apple Root CA
   SHA1 fingerprint: 61 1E 5B 66 2C 59 3A 08 FF 58 D1 4A E2 24 52 D1 98 DF 6C 60


Exits with 0 on success.