How did cameramen manually focus in the film days?
Good quality film cameras have a reflex viewfinder system, where "reflex" means that you're viewing through the lens, and thus you can judge focus by eye. 

To achieve this There are, er were, two basic strategies: 


Split the beam of light from the lens with a prism or half silvered
    mirror, meaning that most of the light goes to the film, and a bit
    goes to the viewfinder.    Advantages: no shutter flicker,
    cheaper. Disadvantages: less light,    both for exposing the
    film, and a dimmer image in the viewfinder (in    low light
    situations it can make it difficult to focus or even see what's
    going on).    This is how my Bolex camera does it.


Position a revolving mirror between the lens and the film plane. While the film is being pulled down by the sprockets the mirror
moves into place, bouncing the light into the viewfinder, and once
the film is ready to be exposed, the mirrors swings around and the
viewfinder goes temporarily black. It's basically what an SLR camera
does, but instead of flipping up the mirror is rotating.
Advantages: more light for the film and the viewfinder, one less optical element in the light path, ability to vary the angle of the
shutter allowing for fine control over exposure without affecting
the depth of field. Disadvantages: More complicated
(=expensive), flicker in viewfinder (actually I think this is
something of an advantage, because you always could tell when the
camera was running, and also when filming video displays you could
make sure that the balck bar was visible in the viewfinder, and
therefore not visible to the film), some degree of rolling shutter.This is how my CP16 does it, as well as pretty much all professional
cinema cameras.

 the mirror is the thing that looks like a bow tie.


As you can see, both methods allowed the operator to see through the lens while filming, allowing for on-the-fly focusing. Video splits also allowed a live video feed to be set up, meaning the focus puller (and director etc.) could have a monitor, but this was relatively late in the piece. A video split is basically a video camera bolted on to where the eyepiece goes, or sometimes with a separate beam-splitter. Until video splits came along the focus puller checked the focus points in the rehearsal, made marks on the floor with camera tape, consulted their cinematographer's manual for the depth-of-field charts, and eyeballed it.

You'n whippersnappers don't know how easy you gots it. Now get off my lawn!