closed keg fermentation - is it taking place?

There won't be any gargling. You can always pull the PRV quickly to see if there is any pressure building up... but then you are losing CO2 that isn't going to make it into the beer.

Most kegs still need a blast of CO2 to properly seat the lid. The slow rise in pressure while naturally carbonating often isn't enough to seat and seal the lid. At this point it is possible that your CO2 has been escaping the keg.

I assume you have a CO2 tank and regulator based on your comments on a different answer. Looks like you're going to have to naturally carbonate anyway, so why not pull on the PRV to see if there is any pressure. Hook up your CO2 tank and blast in 20PSI of pressure to determine if the lid seals. Then crank it back to your intended carbonation/serving pressure, chill the keg and wait a few days before checking again. JUST MAKE SURE THE LID IS SEALED. Otherwise you'll just bleed out all the CO2 from your tank.

I wouldn't expect to hear anything. It's a closed system, the CO2 is dissolving into the beer, not bubbling to the top - as there is no pressure difference for the bubbles to "gurgle-into" (your bottles don't gurgle right!?)

It would be absolutely normal for this to work fine. Even clear beers can have ~100 ppm of yeast cells. So unless this beer was super-high gravity, and the yeast were at their alcohol limit, everything will be OK. Even if it's at super high gravity, it will probably still work, just take a little longer.

Yes in theory it is. but no real way to tell, until serving time.

one way to tell is either hook up a regulator to it and get the pressure, or press the gas in valve without anything hooked up to see if its building up pressure.