At an ASDE-X equipped airport like DCA, there is surface radar that the tower manager, for example, might be keeping an eye on. This is just some screen with a high-res view of everything on the ground. Then you "notice" the mistake, jump out of your seat, run over to the radio, and yell at both of them to stop in place. That's basically how it works today.
On a clear day though, it's probably just people looking out the window of the tower (maybe with binoculars) and realizing in horror that there's been a mistake. The "noticing" is just intuiting the trajectory of the two moving objects...
On a clear day though, it's probably just people looking out the window of the tower (maybe with binoculars) and realizing in horror that there's been a mistake. The "noticing" is just intuiting the trajectory of the two moving objects...