Customers who have never checked into a class will appear in the Customers - Details report. The report has the option to filter on Retention and Activity segments.
To find ‘New’ customers without a check-in, set the date filter to be the last 3 months and select 'More Filters' and set the Retention Segment filter to ‘New.’
If you are looking for customers with no check-ins and who created an account over 3 months ago, set the date filter to be past the last 3 months and set the Retention Segment filter to ‘Dormant.’