You would use a reference source, which may be paper documentation provided by the tool vendor (e.g. API documentation with usage samples), online documentation

You would use a reference source, which may be paper documentation provided by the tool vendor (e.g. API documentation with usage samples), online documentation provided by the tool vendor, sample source code provided by the tool vendor (including illustrative sample applications for reference was quite common), or third party reference books. Since documentation was much more necessary than nowadays, it was more thorough and generally of a higher quality than now, though of course not always perfect.