Following script gives you all user database users, that do not have a server login mapped or where database user name does not match server login name. This is follow up post to How get all users and their role mappings from all databases? I posted few days ago.
IF EXISTS(SELECT * FROM tempdb.sys.tables WHERE name like '#Output%')
DROP TABLE #Output
GO
CREATE TABLE #Output (DatabaseName VARCHAR(255), UserLoginSID varbinary(128), ServerLoginName VARCHAR(255), DatabaseUserName VARCHAR(255), UserType VARCHAR(50))
GO
sp_msforeachdb '
INSERT INTO #Output
SELECT ''?''
, DP.sid
, SP.name
, DP.name
, DP.type_desc
FROM sys.database_principals DP
LEFT JOIN sys.server_principals SP
ON DP.sid = SP.sid
WHERE DP.type_desc IN (''SQL_USER'',''WINDOWS_USER'')
AND DP.sid IS NOT NULL'
GO
SELECT *
FROM #Output
WHERE ServerLoginName IS NULL
OR ServerLoginName <> DatabaseUserName
ORDER BY DatabaseName, ServerLoginName
GO
This post is cross posted on my SQLCAN Blog, MSDN Blog, and SQL server Consultation blog.
SQL Canada (SQLCAN) 
That’s perfect! The only thing I did was tweak the query from #Output to filter out ‘dbo’ and ‘guest’:
SELECT *
FROM #Output
WHERE (ServerLoginName IS NULL
OR ServerLoginName DatabaseUserName)
and DatabaseUserName not in (‘dbo’, ‘guest’)
ORDER BY DatabaseName, ServerLoginName
Thanks Mohit!
I think that is a fair suggestion for guest. However for DBO, I would recommend leaving it in, because there can be possibility that login mapped as Database Owner has been deleted. This will let you know, if dbo maps to NULL, then your database owner property needs to be updated.