[SalesForce] Apex Test Class Assert for Void

How can I build an Apex test class to validate a class method that has a @future HTTPRequest setting? The @future method returns a void, but I can't test a void value with a System.assert[Not]Equals…

The change to the system that triggers the future async call happens on user interaction, so any changes to the system would already be set. The scenario is that the user changes an sObject field (Account Owner) and that change triggers the async http request:

trigger AccountOwnerUpdate on Account (after update) {
    for (Account a : Trigger.new){
        if (a.OwnerId != Trigger.oldMap.get(a.Id).OwnerId) {
            List<Account> aId = [Select OwnerId From Account where Id = :a.Id];
            List<User> oName = [Select Name from User where Id = :aId[0].OwnerId];

            String newOwnerName = EncodingUtil.urlEncode(oName[0].Name, 'UTF-8');
            String accountId = a.Id;

            AccountOwnerUpdateTest.remoteUpdateAccountOwner(accountId, newOwnerName);
        }
    }
}


global class AccountOwnerUpdateTest {
     @future(callout = true)
     public static void remoteUpdateAccountOwner(String accountId, String newOwnerName) {

        HttpRequest req = new HttpRequest();
        String endpoint = '[My url]/'+accountId+'/[param]/'+newOwnerName;

        req.setEndpoint(endpoint);
        req.setMethod('GET');

        Http http = new Http();
        HTTPResponse res = http.send(req);

   }
}

@isTest (seeAllData=true)
public class AccountOwnerUpdateTestClass {
    static testMethod void validateOwnerUpdate() {
        Test.startTest();
        Test.setMock(HttpCalloutMock.class, new MockHttpRequestResponseGenerator());
        AccountOwnerUpdateTest.remoteUpdateAccountOwner('id','owner');
        Test.stopTest();
// System.assertEquals([?])
    }
}