paulh41086464 .. the Segment & Overlap plan is usually free and will give you an idea, you can then see the overlap between your audiences against their traits in the Audience Reports - Segment-to-Trait Overlap Report
You can then decide if you want to go ahead and select the Activation plan as well.
Hope that helps.. please note you'll need a reporting license to access those Overlap reports (you'll need to request that from Customer Care)