Do you have a particular context in mind? Unless you know the person it's not easy to tell whether they are a bhikkhu or a samanera (unless they are obviously under 20 years old), so it's somewhat of a moot point in most cases. If you do know them, you can ask how to address them...
I don't know of a Pali form of address, but Thai people use the term naen for a novice. This word is used to identify one of the letters in the Thai alphabet: http://www.thai-language.com/let/179
[Thai has an official list of words for identifying letters, along the lines of: "A is for apple".]