VZW probably could figure out who's tethering and who isn't.
The only thing we can tell you are the facts:
Yes, tethering for free is against the TOS you signed when you got your phone.
Yes, Verizon is making it harder to get working tether.
Yes, it still works on the latest gingerbread leaks even with the work that VZW has put in, if your settings are right and you're using the right APK.
No, not a single person has yet gotten any stop tethering or we'll charge you notices from Verizon.
No, there is almost no conceivable way they'll start charging people without giving them notice ahead of time to stop. That's what the other carriers did when they told people about the monitoring.
No, Verizon will not send a bunch of men in black coats to your front door to force you to stop.