It really depends on how you set up your CRM to distinguish customers. For SFDC, Marketo has visibility to the Account, so if there is a field on the Account that denotes "Customer" (and is visible to your Marketo sync user), you can set up a data management campaign to look for that info and update the "Is Customer" field in Marketo. Or, you can have your CRM administrator create a workflow to update the associated contacts on an account with "Customer", you will just need to make sure that the field is visible to Marketo.
How do you currently have things set up?
The challenge is that there is no CRM administrator in-house and not a very deep institutional knowledge of how/why/what is configured due to staff turnover. So I found various fields that seemed to be meant to designate customers but they were only used here and there. However, good news! I went into the CRM instance just now and found an Account Field ("Type") that is set to the value "Customer" for customers - and, fingers crossed - it seems to be used consistently (yay!). I sent my client a list and am having them verify.
Thank you for your input!