If you ever don't know the kanji for someone's name, you should ask them for it. As you can see from JimmySeal's post, there are a multitude of different ways to write the same name in Japanese. It's always easier to go the other way (kanji -> reading).
One tip, though, if the person is famous, then they most likely have an entry in Wikipedia. You can search there, then follow the link to the Japanese page where it will have their name written in kanji, or you could just search Japanese Wikipedia with the hiragana and you should get the same result. Sometimes Google searches work as well, as profile sites will have the person's name in kanji and rômaji.