A. For SQL 6.5 and earlier run SQL setup and choose 'Configure Server' and then 'Network Support'. This will tell you what net-libs are installed and allows you to configure new ones.
For SQL 7.0 and above there is a "Server Network Utility" installed in the SQL Server program group that performs the same function.
For all versions of SQL Server you can check the SQL errorlog for "listening on" lines which tell you what net-libs are being used.