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?

>>1
I've never used OpenGl, but wouldn't you need one corner and a size (or two corners) to draw a rectangle? If you can't work out it out from those you have no business programming.