You would know this by reading the manual pages for the broker plugins.
Oanda:
Oanda works around most NFA issues on their side of the API, so the NFA flag needs not be set.
IB:
The NFA flag must be set for IB accounts.
This is because issuing Trades (NFA=off) to the broker vs. issuing Orders (NFA=on) to the broker are completely different endeavors.
https://zorro-project.com/manual/en/brokerplugin.htm
