private boolean extensionIsRegistered() {
        try {
            String extensionUrl = connect.endpointForResource("integration-test-rest-extension/registered");
            Response response = connect.requestGet(extensionUrl);
            return response.getStatus() < BAD_REQUEST.getStatusCode();
        } catch (ConnectException e) {
            return false;
        }
    }
A simple integration test to ensure that REST extensions are registered correctly.
connect/runtime/src/test/java/org/apache/kafka/connect/integration/RestExtensionIntegrationTest.java