Foxes, both male and female, are very similar looking. Even through different subspecies of foxes, it isn't easy to tell them apart. One of the most common type of foxes--the red fox--looks almost identical regardless of the gender difference. There are a few ways that you can tell a female from a male by comparing size in length and weight. And there is one irrefutable way that you can tell the difference. Just be aware that foxes of different ages differ in size as well, and coat length changes throughout the seasons and should not be relied on as a gender indicator.
Measure the length of the fox. Adult male red foxes are typically 54 to 78cm in length. Females are 56 to 74cm in length.
Weigh the foxes. Females are 4 to 4.5kg in weight, and males are 4.5 to 5.4kg. Also, males tend to have broader heads with longer, narrower snouts than females.
Look at fox genitalia. This is the sure-fire way to tell the difference between male and females. Males have a penis sheath; it is small but you will recognise the absence of one in the female, if you see the male. The female's genitalia is called the mammae, and is difficult to see through her fur.