How is latency determined for a monitor?

Serveral people on another site have complained about about lag time of a HDTV (Sharp Aquos LC32D6U) that was reviewed by one of Toms Hardware Publishing sites (Den Guru). One of the reasons that people have purchased that TV was they wanted a good HDTV to be used for PC gaming with the Xbox 360 and the review indicated that the TV had response times between 6-15 ms. Some people that bought it stated that it had absolutely no motion blur or ghosting but had a lag time when playing fast games while others claimed it had no lag time. The following is a link to that review.

http://denguru.com/2006/01/20/are_ [...] age19.html

Since I don't know if the amount of the lag that people are complaining about is 16 ms or 250 ms (as some say), it is hard to understand what could be causing the delay. Maybe lag time like ghostiing or motion blur is more sensitive to some people.

How does THG determine the latency of a monitor? Does it subract out the transfering data to the monitor, any buffering that may be performed by the monitor, and any scaling that the monitor may be doing to come up with the latency time?