Doctor Who zips all the way up and down through time, popping in at any time and place. If you don’t have a time machine to follow them around with, it should be impossible to keep track of which incarnation was where. And yet, the Doctor’s enemies somehow manage to do just that, with the Daleks being accurate enough to determine he was on his last regeneration on Trenzalore.
With the Daleks, I suppose you could say that they’ve studied the Time Lords well enough to detect the, oh I dunno, chrono-telomeres or whatever that actually determine whether or not a Time Lord has any regenerations left. shrugs
I mean two hearts would do it in most cases. Or just DNA actually.
This would most likely be the in universe answer. Some techno babble said extremely fast that you never hear again for 50 years.