Most mobile web access systems run through an explicit or transparent web proxy to downgrade image quality (=> reduce size) at least. Filtering on user agent would be trivial, though easy to work around.

Just periodically run a log scanner for requests for SWFs, etc. Lots of simple non-constant log scans will reveal this information.

I will use VPN or HTTPS proxy and will sue you if you will use that information for non-technical purposes.

If you're using a VPN, that ALSO is a dead giveaway protocol wise.

Good luck suing them. They offer tethering for a price, you're not paying it. I think the chance you'll get away with invasion of privacy on something that they technically should only see from flagged account is about 0.