as title says, Thanks.
I’m unaware of anything built in, but there’s nothing to stop you from firing your own event when you open the window.
Or are you trying to detect when a window in a 3rd party plugin/package is opened?
of course I can use my booleans, but I have many (5 more) windows, and I don’t want to repeat if and boolean to all of them.
Of course you don’t, because that would be a terrible design. 
This is why something like an event is desirable. If you’re not sure what an event is, I strongly suggest reading up on the concept. Basically, you have a list of interested objects somewhere and function which is called whenever something specific happens which gives everything in that list an opportunity to react. I’m not sure about equivalents in Unity’s JavaScript or Boo, but C# and many other languages have very nice built in functionality to support this style of programming. (In C# there’s delegates and System.Action. At Uni I was taught to do it manually in Java using interfaces, though I assume there are better ways now. In C there’s the good ol’ function pointer. And so on.)
you mean use delegate and event to GUI.Window method?
how?