How can I tell which Stripe account a specific order used?
The Orders page (Admin/Store/Orders) has a Stripe Account column, and you can filter the list down to orders that went to a particular account. It's also included in the CSV export, so it's easy to reconcile against your own records.
This column and filter are only visible to store managers and admins looking at everyone's orders — a parent looking at their own order history never sees which Stripe account was involved.