For them to allow you full access to putting a finger inside. If you tried once and she pushes your hand away or moves away, just ignore trying again for a couple minutes and continue with foreplay and daty. Pay attention to the clit and try saying things to get her turned on and "comfortable." As a woman once told me (outside of the hobby) if "a woman doesn't want to cum, no matter what you try she will not" and if she is not cumming she isn't that turned on and your chances of fiv is slim.
And remember guys, not because a woman is wet, means she orgasm.