How do I verify which Salesforce instance is tied to my Marketo account?

Elizabeth,
That doesn't seem possible before I've actually synced Marketo and SFDC. 

I see the screen you're referring to on my live account, where I've already synced, but not on my Marketo Sandbox account, where I haven't.  Below is what I see in my pre-synced Sandbox account;



It's not clear what to click on to get visibility on which SFDC account is linked.

Hi Matt,
Before the initial SalesForce sync, there is no SFDC account linked to the Marketo instance. With that in mind, there is no way to determine which account it is. In this case, your best bet would be to go into your production instance and see what the sync user email address is there, and, using the process of elimination, determine what Salesforce account isn't associated with a Marketo instance.

Thanks Elizabeth,
I know each Marketo instance can only be associated with a single Salesforce instance but can a single Salesforce instance be linked to FROM two Marketo instances?

For example, if my live Marketo instance is pulling from the salesforce account marketo1@example.com, would it be possible for my sandbox Marketo instance to also pull from marketo1@example.com?

Follow up, before the intitial Salesforce sync can I determine which account Marketo is pulling fields from?

Thanks again,
Matt