How to know when meringue has been over-whipped? I have seen many recipes, SO's and videos where the baker/chef/etc put egg white in a machine and whip it on high speed for anything up to and over 10 minutes.

I have been warned about over whipping my egg whites when making meringue...

Though I will start adding sugar from the stiff peak stage, how do I know if I have over whipped the whites?
The sad fact is: you usually know it when, half an hour after you are done whipping it, it floats in a puddle of liquid. 

The problem is that it actually continues changing after you have stopped whipping. So, you really have to learn what the previous stage looks like, and stop whipping when that is reached. That's why I don't whip to really stiff peaks myself, instead I whip to soft and continue a bit, but stop before they have crossed the border to stiff, and when I let it sit, I have stiff. 

This is assuming that you are making French meringue. If you are making Italian, the hot syrup stabilizes the proteins so they don't expell the water afterwards, and it reduces the volume, so whipping it longer is OK there.