I was watching Tour de France with my father (who is a cat 3 racer), and he claimed to be able to estimate how fast the riders were going by observing the cyclist on the screen. I didn't believe him at first, but as we watched, and the screen showed their speeds, he was fairly accurate. How is this done?
This is not that hard for someone with a lot of riding experience. Often times you can tell the gear a rider is in from the TV. Then you can see the rider's cadence, and the road grade is often given. So it's fairly easy to guess the speed to +-2 mph. Plus, on a mostly flat road, the peleton will be riding at somewhere between 24-27mph, unless they are trying to chase down the lead group. If the lead group is gaining on the peleton, you know they are riding somewhere north of 24-27. And the opposite would be true. At the end of the sprint, the lead riders will be somewhere between 30-35. Given all this, it's not hard to fairly accurately guess the pace of any individual rider, at any given time.
He's a good guesser based on the road grade, their visual appearance of exertion, how tight they're riding and how quickly background objects are moving by...and mostly form experience I'd guess. unless he's dead on he's bound to have a "good guess" because they are riding withing a pretty predictable speed band.
If all you knew was the grade and distance from the finish line you'd be able to make a good guess right now without even looking at the riders.