Normally readers know this through the reading list.
Readers often scroll their reading list to check if there's any story that is updated or has not yet reach the latest chapter. Thus, if they like and want to read your story after seeing that the chapter they are on isn't the latest, they will definitely check it out.
If you really need hard facts, after posting a chapter, go and check back on your readers under "Statistics" after a few hours. You can get a feeling of how fast people do bother to read your story every time you update a chapter. You know this easily especially when the same username keeps popping up every time. The only exception is those who like to binge read a few chapters at a time, but the majority I see tend to read every chapter pretty quickly.
However, some authors do frequently post their chapter updates in their profile, so that helps too.