OpenGL / C++

Sup /prog/
I'm currently trying my way around OpenGL. I've drawn a simple Rectangle and I also know how to determine the current mouse position, when the left mouse button is clicked.
Now how can I found out the window coordinates of my rectangle so I can check if the user does a left click inside the rectangle?

>>4
If you're doing 2d, you can adjust the scale to work in window pixels. Otherwise, do some math.