I am using python library.
oauth2_client = oauth2.GoogleRefreshTokenClient(client_id, client_secret, refresh_token)
adwords_client = adwords.AdWordsClient(developer_token, oauth2_client, user_agent,client_customer_id=client_customer_id)
customer_service = adwords_client.GetService('CustomerService',version='v201702')
customers = customer_service.getCustomers()
print ('You are logged in as a user with access to the following customers:')
for customer in customers:
print ('\t%s' % customer['customerId'])