All I can say is Experience, I use the schematic as my guide a well designed schematic can tell us layout guys what should be close and what should be seporated. In reality you get all sorts of crap... and have to use experience and recognise certain blocks of schematic - switch mode power supplies = noise! think about component seporation and not running tracks directly under. Don't mix analog and digital components seporate where possible, Buffers need to be placed on the right end of the trace... if there's something new a placement review will throw up most things and it can be put right fairly quickly.
Like I said I don't know how it works but I know how to place and track components to make sure that the board will function properly.
I did the layout for a 10KW Battery Charger recently with no input from the engineer and he was surprised how good my first pass placement was and we only needed to tweak a few component which I did in about 10mins while he was there. The way I see it a good layout guy can interpret the engineers intentions and get something 80-100% right most of the time. Let face it you engineers have better things to be doing with your time, like adding extra LED's