Good question! Accuracy of themes is subjective. We run several studies where we used opinions of several people on what the themes should be and then compared the overlap between people with the overlap Thematic has with each of them. This is also referred to as "indexing consistency" in Information Science.
In practice, some customers have read samples of responses and expect certain themes. Others have manually tagged them in the past. They compare their results with their fundings and can see straight away if our results are accurate and reliable.
You can check specific examples in our white paper: http://www.getthematic.com/net-promoter-score-verbatims/ (Don't need to download it, the bulk of the results is published on the page.)
In practice, some customers have read samples of responses and expect certain themes. Others have manually tagged them in the past. They compare their results with their fundings and can see straight away if our results are accurate and reliable.