How to identify change/update in Application Server

Hi Experts, Good Day.

I have a requirement to create Service Entry Sheet (ML81N) by capturing the PO data from a file placed in Application Server.

My user will updated the file with the PO details after completing the Service, and that data should be captured and SES has to be created. Here, I cannot estimate or guess when user updates the file. So, I need to capture/identify the updates in that file (placed in App.Server) and run the process of Service Entry Sheet.

So, any of you guys suggest me methods to capture/identify the updates in a file placed in Application Server.

Thanks in Adv.

-Vijay