He said that when he was doing exploration in difficult, silty conditions, he always prefered to be solo. He felt a buddy was more likely to be a liability than a help. The next day he went to the depths of the cave with Shane.
As the video shows, he told Shane to stop at a certain point, and that is the point that Shane exceeded his limitations by going on. My speculation is that Mike knew that Shane was exceeding those limitations, and by telling him to stop there, he was subtly (not blatantly) telling him, "Going further is, in my opinion, beyond your limitations."
So, perhaps one way of knowing you are going to far is recognizing when more experienced people give you subtle hints that they think you are going beyond your limitations but don't want to be impolite enough to spell it out clearly.