I'm wondering how VZW can really determine this outside of going to websites on a computer the Droid can't use (which with flash would seem to drop drastically in number) - and some people do full HTML browsing on the Droid so that doesn't cut it, either.
Verizon can analyze your data type and destination. If you're downloading Java applets, netflix streams, playing World of Warcraft, etc. then that's how they could find out.
The way to be sure that they can't see you is to use encryption and a proxy when tethering.
PS: not saying verizon does or ever will monitor for tethering, but they can unless encrypt/proxy
WRRROONNNNGGG!!!!!
They can monitor your data usage still even with proxy becuase you are using your MIN and MTN to access verizons gateway. They can pull up at that point how much data you are using, which again, if it taxes the capability for others to access the network. And they deem it a liability for their capability to provide service for others, expect a good ol DEEEEKE!!!! and heave ho from their services.
But again, they are not going to care for right now, because they are not charging for tiered data. Even if you are on unlimited and they are moving to tiered data, and you are taxing the network. You will face some trouble.