If your hot water starts to smell like sulphur, that can also be a sign. Especially if it's worse when you don't run it for a day or two.
I made it a few years before I learned it. When I was a kid we hand-washed, and it took a few years of renting before I caught on - not because of any labels or the like, but because of my troubleshooting a smell.
It's not obvious what it is if you don't already know about it, and if you don't install it yourself...
It's absurd that building codes don't require some sort of inspection or access to the vent if it's not a simple straight-through-the-wall affair.
The problem is that EA will still squat on the IP for the indefinite future, so nobody gets to do anything with it later.
Oh, there's so many other uses for it if you put your mind to it...
The HTV vive has this too.
It's very useful for cleanly exiting SteamVR if your controller batteries run out, if you don't want to just take the headset off and use your normal peripherals instead (maybe there on the other side of a wall? Shrug.)
obsf4, meek, SnowFlake, and WebTunnel all say hello.