How can I see customers who have been refunded?

We don't currently surface that in customer lists, as refunds are really more of a transaction-level entity (i.e., one customer could have multiple refunds). Instead, we do provide Scheduled Data Exports which are a log of each transaction, and there is a refunded_at field which you can use in combination with the rc_original_app_user_id field to determine which users have at least one refund.