Yes, they can detect Tor use. ISPs in places like China, Iran and Syria actively block connections to Tor relays. I am not aware of western ISPs engaging in widespread monitoring of Tor use, although there may be specific LE requests for it. In the United States alone there are 75,000 daily Tor users, and maybe a quarter million connect in a given month. Seems to me it would be exceedingly difficult to watch all those people with very little return on that surveillance, since you don't know what they are doing on the other end. There have been specific cases where the FBI used a person's Tor use as evidence, but they were also monitoring activities on the other end. http://arstechnica.com/tech-policy/2012/03/stakeout-how-the-fbi-tracked-and-busted-a-chicago-anon/ Also, there are ways to make it more difficult to detect, such as using (obfsproxy) bridges or VPNs.