Your question is not fully clear to me. Are you talking about opportunity fields that are used as constraints is opportunity related filters / triggers?
I am referring to a constraint within the field. For example, in the field "Opened Email" there is the option to Add Constraint, such as Date of Activity or Min. Number of Times.
For some of our custom fields we have other "fields" that manifest themselves as Constraints in Marketo, they are not actual fields. Is it possible to see if/how these are mapped from Salesforce?
Hope this was clear...
The constraints are added by Marketo for Custom Objects in SFDC. I do not think a standalone field from Salesforce would have constraints showing up in the filters of Marketo.