Before StackOverflow you could still find your information on forums and newsgroups.
Before the Internet took off it was a much different situation. You'd read many books of course, and to fill in the missing gaps you could read magazines or talk to other people at conferences. IT conferences in those days did make much more sense than they do now, imo.
There was a time before forums and newsgroups when all you had was magazines.
For many Dr. Dobb's (http://www.drdobbs.com) was the only real source of programming information they had. Those with more of an academic background had access to the books by Knuth and others, but those were hard to come by in those days. Bookstores didn't carry them, libraries didn't have them, and Amazon didn't exist so you couldn't buy them special-order even if you wanted to.
I learned C from a book and open-source software I could hack away on that was also written in C. I learned C++ from the Borland manuals that came with their compiler. I learned Perl from of the first O'Reilly books ever made. Newsgroups were only so useful, you'd have trouble getting answers to basic questions in a timely manner. Most were more interested in more high-level discussions like how to evolve the language.
If you wanted mentoring you could take your chances on IRC. Most of the time you'd have to endure an unofficial hazing ritual where they'd ignore you, then ridicule you, then finally, if you hadn't run off in disgust, begrudgingly answer some of your questions. Getting a straight answer to a basic problem was pretty much a social engineering problem.
It was only in the late 1990s that programming-specific sites like Perl Monks (http://perlmonks.org) popped up and filled that niche. That was the precursor to sites like Stack Overflow. I'm extremely glad to have been a part of that community where even the dumbest question would be given a straight-forward, non-judgemental answer, in some cases by the guy who literally wrote the book on Perl.
Before the Internet took off it was a much different situation. You'd read many books of course, and to fill in the missing gaps you could read magazines or talk to other people at conferences. IT conferences in those days did make much more sense than they do now, imo.