Actually it's quite simple how they distinguish crew from passengers.
When you download all those great programs like FABS, the rostering app, platinum card etc, you realize that these are only available to active crew or employees...there is a reason for this. Part of the terms of acceptance for these apps which you agree to when you download them is that you have allowed in return the right for the company to track you. That's how they know who is crew and who isn't. There is more technical details on how they can even identify the individual but I didn't really focus on it because I have never and will never allow any company software on my personal devices. I even use a dedicated laptop which is used for nothing other than access the FOIP.
Next time you arrive at a station, watch the purser's tablet while the cabin crew exit the aircraft and each break the connection with ON AIR. Interesting to watch as the crew connections drop one by one.
I asked a buddy of mine who is a software wiz about it and he told me he could write such applications in minutes...easy to do and done everywhere.