There is no "easy" way for them to tell your tethering, but if they audit your account it will become very obvious at what times you tethered. Personally I don't think anyone should tether just 'because' they can, or do any insane downloading when tethered. If your out and there is no WiFi available then go ahead and do it, but only if you actually need a real PC to do whatever it is you need to, as most simple tasks can be done right on the Droid.
I know most of you don't feel the same way, and that's fine. I just don't want to see Verizon's network get slammed and strained like AT&T's and then we all end up suffering because of it, or for Verizon to start charging more for data because they figured out that people are tethering. I look at it like this, say you run a restaurant that has a buffet.....now you charge a certain amount for each customer right? Most people will have maybe 2 or 3 plates, and some might even have 4. Then you will get people who come in and eat ungodly amounts of food, they will even try and sneak more food out or give some to friends that didn't pay extra for the buffet. Now you have to raise your prices or become stricter in how much food customers are allowed.
Do we want that to happen? Do you wanna pay $40 or $50 a month for data because a select few decided to hog up way more bandwidth then they needed. Do you want Verizon to start monitoring what you do with your data and how much of it you used. Do you want them to start enforcing a monthly cap on data? I know I don't.