[support] hook_submit()

Dear all, my first post to this great community! I wrote a custom module dealing with a node, with its nice hook_submit() implementation. This function is called both when the node is created and when a node is updated. I need to insert a term in a vocabulary and associate it with the node when the node is created; and I need to do nothing when the node is updated. Which is the way in hook_submit() to understand whether the node is being created or updated? Thanks and apologies in advance as most likely it is an issue I could have tackled myself, Davide