A few years ago I noticed one interesting thing in game.
Since then every time I do it, I am called a cheater for it.
I haven't met anybody mention or know about it yet so I decided to provide this quick tip.
If you look at the door of a WC cabin you can notice a small hole under the knob.
When there's someone in the toilet ( no matter if he is standing right behind the door's lock or even if he is up on the toilet seat ) you'll see this little hole as a black pixel.
You can easily check this at the toilets in Vertigo with a friend as they are two side by side so it is actually pretty obvious to notice when one of them has a guest.