We have a checkbox called "Push to Marketo" and we use code and a controller in Salesforce to determine whether the record should in fact sync to Marketo (You'll need a Salesforce developer to set this up - we have loads of criteria to check first). However you can use Salesforce workflows that determine some basic criteria for pushing into Marketo as well. You will need Marketo support to set the checkbox up for you
An example is we have multiple brands and some of those Brands do not use Marketo, So if Brand is Brand A on creation or edit then check the box, if Brand B then the checkbox is not checked. You also need to allow for the reverse, If Brand A is changed to Brand B uncheck that box and stop the sync - I do a clean up of data that is no longer synced when the criteria isn't met in Marketo (that's all automated too)