5mile- after all that, I don't think your question was actually answered. I fly another from Boeing's stable, but the system is probably the same. There are no indications on the transponder itself, but most systems are covered by an EICAS -engine indicated control & alert system (or something like that). On a screen that has engine indications etc., various status and alert messages requiring attention may appear. They probably had come up in orange or white letters 'Transponder Fail' and 'TCAS Fail' (or maybe the took a minute to deduce that if the Transponder had gone, so had TCAS, so they'd better mench it to somebody). We carry a list of all the facilities needed for RVSM, so if it occurs to people to look they can deduce whether they qualify.
Not so funny when you are flying back from MIA with no RVSM. You are firmly excluded from RVSM airspace which means no higher than 28,000'. Rather than expand the area of protection around affected aeroplanes, they are excluded from flying in that airspace and possibly from getting to their destinations.