I'm currently in a similar boat as yourself.
Before my current role I too was asked by Sales for a list of Users sending SFDC emails and Sales Insight emails. We were planning on turning off the SFDC email option and we didn't want anyone to get caught off guard in the change (notification were sent etc). At the time the only thing that I noticed was that any email sent from Sales Insight added an Activity with the subject line starting with, "Was Sent Sales Email". I used this fact to determine the difference between the two.
I've been in a new more Marketo focused role for a couple weeks (getting training tomorrow) and I just came across this.
Go to Analytics > Sales Insight Email Performance > Go to the Setup Tab
From here select the time frame you are after and change the Group Emails by from Email to Sales Rep.
Now when you go to the Report tab you'll see a list of all the Marketo Activities done by each user during the frame you have selected.
What I'm trying to find out now is how do I get a more detailed view of the above report at the individual Lead level?
I'd like to be able to provide Sales a list of Opens and Clicks for the previous week so they can follow up accordingly without having to dive into each individual to see the results.
Hope that helps.