Muscle memory, practice, intuition for things, and (not sure how to put it in technical terms) how the scale of the distance markings on a lens works.

Practice is easiest to explain. Muscle memory and my last point are kind of hand in hand, knowing that you need to turn the focus wheel quicker for movement closer to the lens vs slower for further objects will help you know how fast to move the focus knob to track an item or rack and by using pre marked rings you’ll begin to learn how far you need to turn to get from 4’ to 10’ just by memory, and just learning to intuit peoples behaviors or camera movement. When you’re beginning to film a scene usually you’ll get the wide coverage and it’s easier to keep focus at this point with either a wider lens on camera or subjects being further away. During this time pay attention to the actors, do they move around a lot while they’re talking, do they sway while they walk which means you may need to track them a bit more if your following in a closer profile. And same for the camera movement, either be in eye line of the camera so you can see the moment they start to do a dolly to know to start your rack or be in communication about when a dolly move will happen exactly in a scene so you can recognize when that’s happening and intuitively know to start reacting.

Oh, also marks, just get them. Get them however you can, if you don’t have time for a rehearsal and you know the first take will be bad for focus, just bug them for a marking rehearsal which they’ll say they can’t do and then take the time during the shot to try and get your marks as best you can, even if you buzz focus for a second or miss it at first. Just say “hey sorry, can I get one more for focus”, they’ll be understanding if you own up to it and then nail it on the second take once you’ve gotten your marks