Depending on your modem the return result of the ATD%s might help. A return on 82 may be of interest.

NexCom 300 Modem Commands

82 FAX CALLING TONE DETECTED
When answering a call, the modem has detected a fax calling tone.

Not sure if this in incomming or outgoing though

AT+FCON Facsimile Connection
This response indicates connection with a Fax machine.

Hope it helps
UnderMine