I had an interview and they asked me about shoulder checking.
This is the first time I heard this term. Share your knowledge on this topic.
After research on shoulder check, I found this blog https://watirmelon.blog/2013/12/10/improving-your-agile-flow/
It’s also essential that as soon as development is complete that the tester and any programmers who worked on the story gather for a quick ‘shoulder check’ or test handover. This often involves letting the tester ‘play’ with the functionality on the programmer’s machine, and running through the now completed Trello acceptance criteria. Any misunderstandings or bugs can be discussed and resolved before the card becomes ready for testing.
I've heard of the term shoulder surfing. I suppose shoulder checkin is just another synonym. I may be wrong though.
But, if they're the same, it means while a person is operating their computer or phone or any device, another person lurking around may be keeping a keen eye on that person to see what they are typing or doing. By doing this they may note the key presses and then maybe identify sensitive information like passwords and misuse them. This is not just limited to passwords. It can be any information which is also available on the display.
This is the general concept of shoulder surfing.
Here are a couple of links to help you understand it better,