Intuition is usually built off previous experience and situations - so if we look at it from this angle, the alarm bells should start ringing when a situation is different to a situation you've played before or a type of player you've seen playing before deviating from what they normally do (not always but more often than not). This can be in bet sizings, whether they are betting IP or OOP, the range they should have in theory, villains position, playstyle and a few other factors I'm not taking into consideration right now such as board texture.
Intuition is a huge part of poker (not as much online as live but still prominent enough)
Theres a book called "Thinking fast and slow" which I'm currently reading which goes into great detail about how our intuition works and our "logic systems" work. It will give you more insight than I could