There is an indicator. Your view starts to bob up and down the longer you stay without sleeping. It's hard to notice if you're moving but pretty clear when standing still.
Edit: She also laughs right as she's about to show up
This game is really good! Although I'd like to mention that there were at least two times where I used the scanner and the ladder straight up did not spawn.
Maybe even happened a few other times when I wasn't using he scanner