Hi Marco,
We don't have an endpoint that will return a list of checked in attendees but you can get all attendees and parse through them to find the ones who have checked in.
You'll want to use the 'GET events/:id/attendees' endpoint. You'll receive a list of attendee objects in the response. The attendee object has a barcode field and the value of that field will be either 'used', 'unused', or 'refunded.' You'll know that an attendee is checked in if you find the value 'used' at the barcode key.