SELECT role 
FROM sys.database_mirroring_endpoints;

SELECT EP.name, SP.STATE,
CONVERT(nvarchar(38), suser_name(SP.grantor_principal_id))
AS GRANTOR,
SP.TYPE AS PERMISSION,
CONVERT(nvarchar(46),suser_name(SP.grantee_principal_id))
AS GRANTEE
FROM sys.server_permissions  SP , sys.endpoints EP
WHERE SP.major_id  = EP.endpoint_id
ORDER BY  Permission,grantor, grantee;

--Principal server result

--Name : Mirroring
--STATE : G
--GRANTOR: servername\Administrator
--PERMISSION: co
--GRANTEE: domainname\mydomain account 

(Grantee filed- SQL service account should display instead of my domain id)

--Mirror server result

--Name : Mirroring
--STATE : G
--GRANTOR: sa
--PERMISSION: co
--GRANTEE: domainname\mydomain account 

(Grantee filed- SQL service account should display instead of my domain id)