provided you can extract user-agent strings from your logs...
For Outlook 2007+ (I haven't worked with Office 2003 in some time, I can't remember) they each have their own User agent, so you can look that up via google for more details. I find sites like user-agent-string.info useful (or any others, I have no affiliation with that one but seems to come up first when I search).
I know that Office 2007 will include "Office 12" somewhere in the string, the end normally. Here are some examples with Office 2010:
Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 6.0; Trident/4.0; InfoPath.2; MSOffice 14)
Microsoft Office/14.0 (Windows NT 5.1; Microsoft Outlook 14.0.4536; Pro; MSOffice 14)
You can see that MSOffice 14 is what you'll see for Office 2010. You can probably find more. I know that for a customer of mine at work, he has a ton of Mac users that connect to his exchange server via RPC over HTTP, and when monitoring we discovered that you can even find the specific version (I'm guessing to take into account certain major updates? I'm not an MS guy) of MS Office 2011