Who is using a SQL Server database user credential?
You could use a logon trigger perhaps, going forward (see here and here). 

For historical information, by default, SQL Server doesn't even track successful logins (though this can be turned on - however it means you will need to parse the SQL Server log for some little bit of information). So while you might get some information (such as which workstations were using which logins) from the default trace, it would rely on them performing actions that get logged there (and would only go back as far as your default trace data goes anyway).

Also see this answer.