Is there a signal from the ALTGX indicating the CDR is locked to the incoming data?

The answer is that there is no signal that indicates the receiver is locked to the incoming data stream. The signal "rxfreq_locked" indicates that the CDR is currently in lock-to-data mode. This is why manually asserting ltd will force this signal high. The only way to somewhat determine if the receiver is locked is to monitor the "rxfreq_locked" signal. If it remains asserted for a period of time (meaning the receiver remains in lock-to-data mode) then you can assume it is locked. If the signal deassert, you are not locked to the incoming data stream.