I had to check the status of a webMethods task for a test.
To do this, I created a new service on the Integration Server. The input for the service was a couple of search fields, an id and a task type, and the expected status of the task. If the task was found and the status matched the expected status, the service returned 'true'. Otherwise, it returned false. Then I set up a web service interface to the webMethods service and had DevTest call that as a normal web service call.
You could set up a 'webMethods Integration Server Services' step to call the webMethods service 'pub.task.taskclient:searchTasks'. Then you would have to check the check the results, an array named '/tasks', find the correct task and verify the status. It seemed easier to let a webMethods service search for the task and verify the results.