In my case although things were going very well on the surface, there were a few signs, most of which could have gone either way - one in particular was that her phone was busy but she left it through in the kitchen and dealt with it there when previously she'd have it in the living room with us.
I don't do Facebook, so I never bothered with what she got up to there but she might well have been spending more time on that?
She completely tidied the flat, got our name on the entry and cleared-out her wardrobe. She had talked about changing her style a while before, so I just thought she had finally made her decision and was going to be doing some shopping.
However, the clincher involved a shirt of mine - She liked it a lot (on me and off) and would often wear it when lounging about at home, whenever she washed it, she would "freshen" it up again by putting it on during sex, then have me wear it out with her the next day usually ending-up with it going back on her again during sex when we got home - she loved/was comforted by the smell on it.
Well, one morning I opened the wardrobe and here it was, washed, neatly pressed and put back in with the rest of my shirts. That's when I knew she was disengaging and it couldn't be long. I was right - It was less than a week before I was done with her for good - I'd previously resolved that she'd not get another chance with me, so was ready to bail the moment she started acting-up.