If I understand fully what you are asking (tracing a client connection), you need to set the following in your SQLNET.ORA file on the client in question:

TRACE_LEVEL_CLIENT=ADMIN

or instead of ADMIN, try SUPPORT for a detailed output... The trace file will get big real quick so keep an eye on it!!

You can also set:

TRACE_DIRECTORY_CLIENT=x

To change the default location that your trace file will get written to.