I believe there is a in and out directory under the faxmaker directory where .sfm files are stored.
Using the Network File Count sensor you can count the number of files that are more than xx minutes old and use limits to notify you when there are more than x files.
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.