This question is a bit tricky because you can respond to it in two different ways. The first is to describe how you would proceed with the testing even though the proper documentation was unavailable. This may involve turning to your previous experience, seeking documentation similar to what you would expect for this product, or simply making your best effort despite the lack of documentation. You could also respond by saying that you would not proceed with the software testing and would instead seek to obtain the documentation necessary for the testing. Most employers prefer the second option since they try to maintain standard procedures and are sometimes governed by rules and regulations requiring them to produce documentation upon request.