really it just comes from a combination of context and practice; chances are if it's a part of a kanji compound, it'll be onyomi (ie 新聞), and if it's not part of a compound, it's probably kunyomi (新しい) In written context, this is probably the easiest way to differentiate which way to read the kanji. Of course, there are several exceptions to this, which you just have to learn to recognize (大人 etc)