Hi! I don't believe there's a way to do this directly in Marketo without bringing in additional data points from a 3rd party.
In the past, I've worked with Litmus to provide that type of ESP analysis. I'm sure there are some LaunchPoint partners who would provide that data as well.
Outlook Desktop includes “Microsoft Outlook“ in the User-Agent:
Your question wasn’t specific enough, though, as there’s also the Outlook Mobile App. And of course in a very multi-device world, someone doesn’t just open emails in one client app, you should assume they have 2 apps (desktop/web and mobile) of equal significance.