use the AUDIT LOGIN EVENT in SQL Profiler?
The AUDIT LOGIN
EVENT in SQL Profiler collects all new connection events since
the trace was started (for example, a client requesting a connection
to a server running an instance of SQL Server). You would use
it for security audit occasionally or 24/7 if you need login history.
Trace tends to be quite large.
SQL Server Event Class Reference
Step-By-Step: An introduction to SQL Server Profiler