I was lucky that one of the first issues of Compute! I got my hands on had a listing for a proofreader[1], a utility that checksummed each typed line of BASIC and displayed the checksum in the left margin. All listings in the magazine had these checksums printed to the left of the line numbers, so it was easy to catch typing errors.

This was especially useful without any kind of permanent storage. I had to type in my games every time. :-)

[1] Compute! November 1986 issue, page 94. https://ia600700.us.archive.org/6/items/1986-11-compute-maga...