It's debatable if women actually can there are those who say yes & those who say no. Well, proven yes I can. Have to be very comfortable [at ease]& require g-spot stim. It's secondary of a internal release of fluid from a gland; therefore, I doubt it has much to correlate with her wetness during arousal. Some women never experience this. If I am with a person I feel close to & very comfortable; it happens maybe 50% of all times which I orgasm. It's not a guarantee.
I doubt you can cause it--work on the g-spot and work it slowly & see what happens. I hope you read Brandye's post on this. If so here is a bit more of similar answers:
http://www.health24.com/sex/Basics/1253-1265,32986.asp