how to know wat is the data source name.

hi there,
i have got prob connecting to the oracle server through my vb6 client
server application.while setting the dsn entry i dont know the name of
the data source,so is there any entry of it in the server.this is a
prebuilt application which on running gives
"data source not found no default driver specified error "

is the data source name same as the SID value in the tnsnames.ora file?

.