Writer's Fallout Shelter

Let a few people read it at some point. Don't even mention gender or characterization (or they'll be looking too hard for flaws). After they've read it, if they don't bring up the issue, prompt them - "Were the characters believable? How about the women?"

Don't feel bad. Tom Clancy can't write credible women to save his life, but look how successful he is.

Specifically what do you find difficult about writing women characters into your novel?