It falls leaves a ring of gunk and you have floaties on top,usually. Just because it falls and looks done doesnt mean it is, only your hydrometer will let you know that.
Its a good idea to leave it for a few more weeks,before bottleing.
I've been brewing since 2003 and to be honest I have had the Krausen fall and I've had it stay on top. The majority of the time it will fall but because it doesn't don't worry chances are the beer is fine. All the beer I brewed where the Krausen stays on top turned out just fine.
It really depends on your yeast and recipe and temp as to how long it will take to fall. All will eventually fall. If it's been a couple weeks since you think it should have fallen I say move the thing around a little and it probably will fall. Cold crashing works great as well.