Hi Liz,
In SFDC, you can see the Sales Emails you sent by going to the Marketo tab and clicking on "My Emails." If you want to see what Sales Emails were sent to a Lead or Contact, you can bring up their page and there's an "Email" tab in the Visualforce section for Sales Insight.
In Marketo, you can create a Smart List using the filter "Was Sent Sales Email" and adding the Constraint of "Sent By." This smart list would show the Leads who were sent a Sales Email.
You may want to add your voice to an Idea in the Community about additional reporting for Sales Insight:
https://community.marketo.com/MarketoIdeaDetail?id=08750000000HBDAAA4