Back in the day most of the computer magazines had one of those. I think it was Nibble magazine that published several programs for the Apple ][.
IIRC you'd start the program in the background and it would watch what line your cursor was on and display a two digit checksum in the upper right corner of the screen that would update as you typed. Just make sure that number matched the check on the end of the line in the magazine and you were clear to hit Return to save the line.
A different magazine had a similar method, but I believe it provided line-by-line checksums after you were done entering the program, and would also generate a "program checksum" at the end that would match if all lines were correct.
I also remember several occasions where there was a printing problem in the magazine and everyone's checksum was wrong, they'd publish a correction in next month's edition and everyone would cry "THAT'S why I couldn't get it right!" (probably after receiving hundreds of letters in the mail complaining about hours of frustration trying to key it in!) This was frequently due to the magazine omitting a line of code. (all the line checksums matched, but not the total at the bottom)
ahh the good ol days of Human OCR....