There is no objective "distance you see". E.g. on a larger monitor the distance will be higher. You can measure the real distance of two points in your drawing/model (in the drawing units) with DIST.

I think the only reliable way to get the length of what you see on the screen is to place your scale/ruler against the screen. A program could be made to read the AutoCAD window size in  inches, read the current zoom scale in AutoCAD, then figure the display scale. But for what purpose?

Sounds to me your looking to scale your drawing. Set the global scale for your drawing then go to paperspace and then measure your line that will give you the distance of that line of what it would be if you plotted it out. What ever you draw does not change no matter what computer you use. How you set up your drawing is how everyone see's it unless someone changes some of the settings. Hope this helps