How to determine if a user is customer in a Jira Service Management project via REST API

How can I determine via REST API if a certain user key or accountId belongs to a “customer” in a specific Jira Service Management project?

One way is to get all the customers of that project (via this endpoint GET /rest/servicedeskapi/servicedesk/{serviceDeskId}/customer) and then iterate through them to check if the key/accountId matches any of them… which is not optimal at all.

There should be a simpler way. Any idea?