You can check if the task has been created on Wunderlist just by checking for the "Wunderlist" category. If the task has it, it was created on Wunderlist.
After the creation of the task, it should look like this:
As you can see, no category has been assigned yet. A few seconds later (~5), our sync will create the task on Wunderlist. You can see immediately, that the category is added: