Kiya is right.
It isn't always true, but you can usually tell the sex of the author from the way they write characters (Or at least that's how I do it <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />). It is very rare for a man to write women well, and vice versa.
Both sexes understand each other imperfectly - and it shows <img src="/ubbthreads/images/graemlins/smile.gif" alt="" />
True to an extent. Here are some generalizations (and of course, there are always exceptions, and good authors don't fit into this anyway)--
1. Female authors generally choreograph, say, combat more poorly. The more gritty details will not be there. (Keeping this to fantasy genre, most heroines will wield long swords and not, say, battleaxes. They'll cut things up and get cut, but things like getting punched and having to spit out teeth are rarely included.)
2. Bad female authors write male characters as protective, sensitive -- the perfect mates. Bad male authors write female characters as fluffy, understanding, sexy -- the perfect women, with exaggerated feminine attributes. Regardless of the author's gender, romances almost always end up happy.
3. Bad female authors overdo emotions; bad male authors overdo action.
4. Bad female authors write female characters as pure-hearted, beautiful and delicate yet deadly, intelligent and witty yet understanding, can engage in action with the best of them but still have amazing motherly instincts. "Strong" female characters, from them, are male-bashing and obnoxious (but get worshipped regardless). Bad male authors write male characters as authority-defying rogues whose charms no woman can resist.