You can set the RCL_ASSERT_RMW_ID_MATCHES environment variable to be sure. If it doesn’t match you’ll get an error like:
$ RCL_ASSERT_RMW_ID_MATCHES=rmw_connext_cpp RMW_IMPLEMENTATION=rmw_fastrtps_cpp ros2 run demo_nodes_cpp talker
Expected RMW implementation identifier of 'rmw_connext_cpp' but instead found 'rmw_fastrtps_cpp', exiting with 102.
We have ideas for how to make this introspection easier for users in the future, but for the meantime I’ve added this info to the wiki page.