There's a distinct difference in smell, which is the surest first clue, the anatomical shape of the carcass relative to one another being another, you can see this when there is a male and a female hanging with the jacket off. Oh, and the ease of actually skinning a female compared to a male, particularly round the neck area. Other markers include the relative ease/ practical opening of the pelvic arch in a mature roebuck with just a knife, which is pretty impossible on an mature doe that has calved.