In API, linphone_call_params_fec_enabled() returns true if a FEC stream exists, that means that both participants have enabled the FEC feature and that they use video.
Warning: the method linphone_core_enable_fec() returns true whether the payload type for flexfec has been found. It does not imply that the FEC is running, as the other participant may not have the payload for flexfec.
Maybe linphone_core_fec_enabled() could be renamed in linphone_core_fec_available()?