Look at the modified dates of the PST. The date give you a good indication of when it was last accessed. Also, I would check the size of the PST. The PST will usually be the larger pst since the user uses it the most.

the "outlook.pst" in c:\email may the file you want to use.

How do yo open the file:

1. In Outlook > File
2. Select Open > outlook data file?