Who has changed the Custom Field value?

Dears,

Need to know the information regarding how to fetch that who has changed the Custom field value from PWA/MPP.

Example:

There is a PDP (say ProjectInfo):

There is a Custom Field added to that PDP (say Project Status = In Progress).

Now for ProjectA, someone has checked out and changed the Custom Field "Project Status" from "In Progress" to "Closed".

I need to fetch the information regarding who has changed that Custom field.

In which direction should I move: Event Handlers, PSI???? Any other stuff?

Thanks & Regards,

Shravan