Send this request providing, at the very least, the brand_id and currency query parameters having the same values you'd use to create your Purchase. Be sure to use the same API key you'll create your Purchase with; it will define the test_mode setting used in the lookup.

In the response body you'll receive an object with available_payment_methods property containing the list of payment method names available to use with your Purchase (e.g. those codes can be used in payment_method_whitelist field or with ?preferred={payment_method} option of checkout_url).