EDIT: Never mind..there's a 'retrieve application' on the US customs website.
Weirdly, I wouldn't fully trust that. I tried to retrieve my application before I went to the US a few weeks ago. I knew I had a valid one, but the system said it had expired, because for what ever reason, I brought back the one I had prior to my valid one. I had to search my emails to find my application number to double check the validity of the current one.