@Nicky Davis Lack of spermathecae is one thing, but they can be folded in sometimes and not be easy to see. What makes this an obvious and doubtless male is the field of epiandrous fusillae - the light triangular patch in the darker field between the book lungs.
It's the overall appearance, though. Females not only have spermathecae and utereus externus but also a bursa copulatrix that is usually very easy to see and lacking here. They also have a wider and more obvious furrow. There are actually plenaty of differences.