There are some people cross dress not because they like it but because they are simply playing a role as with any wearing any costume…Barry Humphries is an example of this, he mearly has to wear womens clothing (not because he likes to) but because its part of playing his character Dame Edna. Its much the same as a person playing an ethnic part different from their own or an orc or elf or alien.
I see cross dressing in LARP much the same way as cross dressing in theatre…Its a costume, for what ever reason you outwardly want to portray the oppisite sex to what you are…either your character is dressed as the oppisite sex or is the oppisite sex to what you are.
If a player is dressed as a particular sex, treat the character as though they are the sex they are dressed as untill you are totaly clear they are mearly a character dressed as the oppisite sex, It’s realy the only thing you can do besides taking the player aside and asking weither they are obviously a crossdressing character or weither they appear to actualy be the sex they appear to be. If your character is dressed as the oppisite sex and you want to get that across then drop the hints (even to the point of changing your voice back for a sentance then doing the “oh whoops” and going back or adjusting ones self, complaining about the heels etc)…unless of course your going for the “secretly an X” in which case just dont be offended if no-one works it out.
But I would recommend that if you dont think you can pull off a convincing [oppisite sex] then you dont try to be a character that is actualy the oppisite sex.