I suppose the "fax receive" CLI command could print an informative message reminding you that chat must be open, before sending.
Note that the FaxRequestFrame does this:
I don't know if that is 100% reliable; I'm pretty sure that if you open chat and close it, ChatManager will still be running.
// Make sure we can receive chat messages, either via KoLmafia chat or in the Relay Browser.
if ( !( ChatManager.isRunning() || true ) )
FaxRequestFrame.statusMessage = "You must be in chat so we can receive messages from " + botName;
KoLmafia.updateDisplay( FaxRequestFrame.statusMessage );
Which is why there is "|| true" following the check; you'll never see that message.