Most male mammals have visible nipples only they'll be smaller than in a female that is sexually mature.
Look at the anal-genital gap. In males this is much longer than in females.
The best way to do this is to put them in an (empty!) pint pot, glass jar or cricket tub so that you can view their bits without getting bitten fingers.
Sexually mature males are very easy to sex... you wont be able to miss those balls...:whistling2: