There are a couple of ways they can try to detect devtools being opened. As the sibling comment implies, the most popular way is to detect a sudden viewport resize, and you can avoid that by ensuring your devtools are set to open in a new window before opening them.
The only other ways I'm aware of are:
- Detecting the keyboard shortcut, ⌘⌥i or equivalent, which you can avoid by using the browser menu, and
- More riskily, evaluating a `debugger` statement and detecting whether evaluation paused. I'm not sure you could do anything about this one, but it would certainly be obvious to you whether it was happening.
The only other ways I'm aware of are:
- Detecting the keyboard shortcut, ⌘⌥i or equivalent, which you can avoid by using the browser menu, and
- More riskily, evaluating a `debugger` statement and detecting whether evaluation paused. I'm not sure you could do anything about this one, but it would certainly be obvious to you whether it was happening.