After how long can we know the gender of a (wood) mouse?

And, well, how? I know where to look, of course, but how would it look like if male or female, I mean...
Thanks.
Answers:
I must say that I have no idea if I've ever seen a "wood mouse". but, in addition to the link already given by the other person who replied, here are some more pics that are helpful for sexing Mus musculus mice (regular pet mice, as well as house mice and lab mice and so-called feeder mice... they're basically all the same species).
http://www.thefunmouse.com/info/sexing.c...
http://www.fancymice.info/sexing.htm...
http://www.fancymice.info/sexingcait.htm...
Experienced mouse breeders can sex mouse pups from a very young age. If you're less experienced, the earliest you can try to sex the little ones is probably when their fur comes in: you should be able to notice that only the females have small partings in their belly fur (this is where the nipples are).
Hope this helps.
Greetz.
Cyzahhe

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To identify the gender of a mouse, the anogenital distance (space between the anus and the external genital area) is used. Males have a larger space between the two structures than females.
(Check the link for pics)