They can tell based on how the data entry over the network. Abuse or not if they want to come after you they will.
When I start hearing stories of VZW coming after someone who tethers a few times here and there and uses minimal amounts of data, I'll start to worry. I have yet to hear anything of the sort. Nice try with the scare tactics.
I work from home and use my WIFI connection all day so I use very very little 3G data. I've tethered 3 times in the last 9 months. Two times for testing and once when the power went out at my home, so I fired up my laptop. I might use it when I go camping next month. Not only am I not worried, but if big red ever did send me a letter or call me about it. Considering my minimal data use, they wouldn't have me as a customer any longer.