How do I know when my phone is done charging?

Can someone please answer this really quick?

I have not been able to find this info anywhere, not even in the iPhone user guide and customer service even had trouble answering it.

So the question...

I was just wondering how I know when my iPhone is fully charged. I see an icon of a battery in the upper right hand corner of the screen (It is white in color). Sometimes it shows the battery [icon] with a lightning bolt through it and sometimes with an electrical plug in it. Do one of these mean it's charging and the other that it is done charging? Or does it change to green from white? Or what do all these icons and pictures mean?

I'm really confused here and would appreciate anyone who is able to help!

Thanks in advance everyone!

Mike.