Prefacing an answer with the word "obviously," is sarcastic.
Being literal can combat sarcasm, but it requires tenacity. The trick is to ask them to explain their remarks in detail, with a straight-faced demeanor. I have an employee with a sarcasm problem. I'm going to get him to give it up if it kills me or him. Sarcasm is fun taken occasionally in small doses, but as a way of communicating, it's not productive.
She: "Obviously, the invoices are for different services, because they have different dates."
You: "In what way is this obvious? The postings are identical, and a similar duplicate invoice was discovered last month."
She: "Well, that was different."
You: "In what way was it different?"