Hmm, too bad that it's SFTP - FME supports an FTP Watcher but sounds like not SFTP - you could try the FTPWatcher to see if it works though

https://community.safe.com/s/idea/0874Q000000TkyUQAS/detail

There is a suggested work around here, but it's not ideal as it requires a scheduled FME Job to do it. If you wanted something that doesn't need an engine it looks like you might need to look outside of FME to do the polling.