Situation: early 2008 MacBook Pro (4.1) running Mac OS X 10.5.8 shows Airport OFF on startup, and doesn't respond to Turn Airport ON menu bar selection. Ethernet OK. Apple Hardware test shows everything OK, but the Communications pane doesn't list an Airport MAC, only Ethernet and FireWire MACs, plus the IR module (this may be normal, but I don't know).

Question: how can I make sure that the (faulty?) Airport card is causing this behavior, and may have to be replaced, without actually replacing it?

Hmm. After a couple of reboots (I forgot to mention above) that didn't change anything, I prepared an external HD to start up from a clone when I noticed Airport being back up again. Rebooted from the clone anyway, and all was well there too, so no software issue. I wonder if the MBP's Airport card needs to be reseated. Something to check if the issue recurs. Meanwhile, my wife's happy again (she hated the ethernet tether, or perhaps the only chair within its reach).

Meanwhile, while we're on the topic, I still like to know the answer to the question I posed above.

sounds like the airpot is not detected. once the port is added it will stay there until removed by you. go into network prefs and [-] it then [+] it. if the hardware isn't detected, it won't be available to add.

booting off a known good os is the only way to be certain it's not a software/driver issue, but it sounds like you've already been there. so it's likely a hardware failure. Loose cables happen, but are extremely rare in this case.

Thanks for confirming my thoughts about the clone-boot test. I also mentioned reseating the card, but afterward realized that (enough) bad contacts increase the likelihood of triggering a "No Airport card installed" error message, or something to that effect. I just got nothing, which seemed to point at a hardware defect. (I once again remembered the lack of a specific Airport hardware test in the battery of commercial utilities I have.)

But despite all these musings it wasn't anything of the sort, and the card now works as if nothing untoward happened. I attribute it to a 'faulty' reboot. Time will tell.