The easiest way seems to be using the [sys.crypt_properties][1] Catalog View which holds the relationship between Certificates / Asymmetric Keys and signed modules, as well as the usage (i.e. Signature or Counter-Signature).

<!-- language: lang-sql -->

    SELECT SCHEMA_NAME(so.[schema_id]) AS [SchemaName],
           so.[name] AS [ObjectName],
           so.[type_desc] AS [ObjectType],
           ---
    	   scp.crypt_type_desc AS [SignatureType],
           ISNULL(sc.[name], sak.[name]) AS [CertOrAsymKeyName],
           ---
           scp.thumbprint
    FROM sys.crypt_properties scp
    INNER JOIN sys.objects so
            ON so.[object_id] = scp.[major_id]
    LEFT JOIN sys.certificates sc
            ON sc.thumbprint = scp.thumbprint
    LEFT JOIN sys.asymmetric_keys sak
            ON sak.thumbprint = scp.thumbprint
    WHERE   so.[type] <> 'U'
    ORDER BY [SchemaName], [ObjectType], [ObjectName], [CertOrAsymKeyName];

In my test DB, this query returns the following:

<!-- language: lang-none -->

    Schema  ObjectName     ObjectType            SignatureType                        CertOrAsymKeyName  thumbprint
    ------  ----------     ----------            -------------                        -----------------  ----------
    dbo     fnPaymentCalc  SQL_SCALAR_FUNCTION   COUNTER SIGNATURE BY ASYMMETRIC KEY  KeyTest1           0x2333B2FA6AA8004E
    dbo     ModuleTest2	   SQL_STORED_PROCEDURE  SIGNATURE BY CERTIFICATE             CrossDatabaseCert  0x49BA174584C78C878D923690C15898A809CBACDF
    dbo     TestSig        SQL_STORED_PROCEDURE  SIGNATURE BY CERTIFICATE             bob                0x778B3DB4ED981FC27AB301ACE7A1AB8424F64792
    dbo     TestSig        SQL_STORED_PROCEDURE  SIGNATURE BY ASYMMETRIC KEY          KeyTest1           0x2333B2FA6AA8004E

  [1]: https://msdn.microsoft.com/en-us/library/ms189536.aspx