I recently had an activity type of, "SFDC Activity", yet was created by Marketo. When I inquired with Marketo, the response was:
I spoke with a senior engineer in regards to whether or not there are unique identifiers to differentiate between MKTO and SFDC activities. Currently there are no such unique identifiers to help in this regard.
It would be nice if a unique identifier existed - especially as integration between SFDC and Marketo grows and the campaign complexity of each expands.