Some necessary but not sufficient criteria for our intrepid programmer to consider himself a master of his language:

* He's written a non-toy compiler for the language.

* He knows how to choose between more or less semantically equivalent ways of writing the same code because he understands the tradeoffs involved.

* He's explored many or most of the dark and mysterious corners of the language and knows when and when not to invoke them in production code.

* He is familiar with the contents of the standard library and uses it whenever appropriate.

* Knuth is his homeboy.

That's all I can think of for now.