You can use the following development call to trace against the /sparql endpoint:
registry_set ('__sparql_endpoint_debug', '1')
This call traces all SPARQL queries cached or not.
The Virtuoso server has to be started in foreground mode for this to work as it is written to stderr.
So you
would start the server with a command of the form:
virtuoso-t -f -c <virtuoso-config-file>