In ye olde dayes, indeed by hand. That's why there was often extra space after punctuation. In more mechanized times, the operator has to watch for it. Proofreaders are trained to watch for loose lines, rivers, widows, hyphenation errors, and other spacing problems. Those things will be marked as errors in proof. Even with modern DTP tools, typesetters still have to make a lot of manual corrections. Of course, for print, you're setting for a fixed format. You can do a lot of fine-tuning that a browser can't do on the fly.
The idea of extra space after punctuation (especially periods) being a result of printing technology is a myth. Extra space is present in handwritten documents: go look at the US Declaration of Independence or Constitution for well-known examples. People only started shortening the space between sentences to match the space between words very recently.