Since I've used several methods for this, I'll toss in my 2 cents.
Historically, it has been by recorded various means. Technically speaking, it can be done with a stopwatch - and that's how I've done geared and SS KT ITTs. That said - I would strongly recommend using a GPS to track an ITT. A tight GPS track will appease any naysayers about your ride. SPOT use is great, especially for significant others and as a safety net, but it does not have good enough time resolution to record exact start and finishes. Used with a watch it's a good solution.
So, some options are...
stopwatch: easiest, but won't back up your claims.
GPS: great resolution (assuming you know how to setup your unit correctly!), backs up claims. You can share the track with the world, or just with whoever will post the time to verify course compliance. Keep in mind those tracks reveal your entire strategy, those familiar with them can tell you more about your ride than you remember
I do have a track for my 2nd grand loop but only 2 others I know of have seen it. It also has power data...but I digress...
SPOT + stopwatch: good solution and entertaining for others.
ITTs are the best. Enjoy!