I found this query...
SELECT DB_NAME() AS 'database'
,pe.permission_name
,pe.state_desc
,pr.name AS 'grantee'
,pr2.name AS 'grantor'
FROM sys.database_permissions pe
JOIN sys.database_principals pr
ON pe.grantee_principal_id = pr.principal_Id
JOIN sys.database_principals pr2
ON pe.grantor_principal_id = pr2.principal_Id
WHERE pe.type = 'IM'
Here...
http://tenbulls.co.uk/2012/01/30/what-are-the-impersonation-rights-for-my-database-users/