Screenshot of the Help field will prove it but you can get the version from the message header in most cases. Look at the message properties in Outlook - right click on the message in the inbox and choose options (if older versions of outlook) and look for the x-mailer line. This is from 2010: X-Mailer: Microsoft Outlook 14.0, Outlook 2007: X-Mailer: Microsoft Office Outlook 12.0
If you use outlook 2010 you need to open the message, then File, Properties to view the header.
If the sender users exchange server it (probably) won't have the x-mailer header. However, that should point to Outlook, as its the exchange client. OE would use SMTP to send and that should add the x-mailer header that says outlook express.