1. There is no standard way.
2. I would avoid it, as an UML-diagram should not be language-specific.
And Inner classes are java-specific.
3. Why do you want to do it?
a) represent your design ->
avoid any special notation. If the class is private, I wouldnt show it at all, if it's public, I'd represent it like a standard class.
b) document your (already coded) classes ->
If you think that showing this class as an inner class will make a benefit to the reader of the diagram, you are free to extend the standard.
Just put a legend somewhere.