Do you mean that they have enrolled in a free trial but not signed up for the paid version? If so, you can filter them using the filter "Enrolled in > free trial > [Course Name]"
Or, if you need to simply see who's paid you and who hasn't, you can filter by "Amount spent > is equal to > 0"
Note that customers cannot sign up for courses that they're already enrolled in fully, so if you need customers who are already enrolled in the course to sign up again, you'll need to expire their enrolments first. To do that, click on the customer's name to view their user profile and then select an expiry date for their enrolment in your course.
If you have any further questions at all, please don't hesitate to get in touch.