Screen? Previews? They didn't exist at the beginning.
it has been asked how knuth viewed his "first page". it was definitely not on screen. it was most likely printed out on the xerox xgp which was installed at the stanford ai (artificial intelligence) lab (sail). in the book "companion to the papers of donald knuth", don says (pp.58-59) that he'd been "playing around" with this machine for a few years. it doesn't say definitively that the "first page" came out of the xgp, but no screen output is mentioned at that stage for display of more than one or two letters at a time, and that's a terrible way to try to decide that a whole page is "perfect".
you can learn a whole lot more about don and printing (and about everything else that he's done) from that book. it contains an extensive index to all his books and a complete bibliography including all his papers that were in print up to 2011. recommended.
At Stanford, there were a couple of electrostatic printers, 200dpi, made by Versitech and Benson-Varian; output was on rolls of slightly slimy paper since it had to go through a smelly liquid bath. Stanford also had a xerox xgp printer (approximately the size of a refrigerator, or a computer mainframe unit, if you remember those) with about the same resolution that used dry toner, but it was coarser then the toners now used, and periodically it would clump into little grains that were large enough to punch tiny holes in the (plain) paper. Output on these was often magnified to 130% and photographically reduced to smooth out the edges.
The first really hi-res output was to an alphatype crs, with a nominal resolution of 5333 dpi. (This was generated on a crt screen at much lower resolution and larger size, and stepped down through lenses.) The photographic paper onto which it was imaged was on large sheets, each of which could -- just barely -- accommodate four letter-size pages, the size of tugboat pages in fact.
Early issues of tugboat -- look for the entries for "outout devices" -- have lots of examples and technical info.
The earliest mention in tugboat of a previewer seems to be in 1982, at Stanford, for the data-disc displays on a decsystem-10. a previewer running under vax/vms was mentioned in the first 1986 issue. The last 1986 issue announced previewing on an early macintosh, complete with a screenshot -- just look at those jaggy edges!
Times have certainly changed.