Client signups and onboarding is handled by the Client Management service group. In there you can find the endpoint GET /cm/v1/signup/status which lets you select specific client (by ClientKey) and provide a response with details on the onboarding status:
"Message": "Onboarding request complete. Account and client keys have been created",
Where the field OnboardingState holds the current onboarding status.
Learn page - Client Management