EV has multiple ways to identify a PST File.
You can have them "marked", so EV will write a hidden message into the PST file when it is attached in Outlook. EV will look the file owner, as well as the owner of the directory it lies in, I believe.
Have a look here:
http://www.enterprisevaultfaq.com/wiki/EV_Documents_(Index)
You will find a whitepaper named "Migrating PST Files into Enterprise Vault", which is most useful to read in your situation, I think.
Cheers
Michel