You can use Command logging to see the physical source SQL.
Unfortunately the command log contains the logical form of the pushdown query, each souce will be different in how or whether an actual source query is logged. For JDBC a trace log is made to org.teiid.CONNECTOR. You can adjust your log4j settings to see the trace logs. Other possibilities include checking into additional logging options in your drivers, usage of a spy driver, or logging an enhancement request.