I'm happy to give a reference even if I've seen a gentleman only one time.

The fillip to that is that if a long enough time has passed, chances are I'm not going to remember him well enough to feel comfortable giving a reference. If I just saw him a week or two ago though, no problem.

References shouldn't be viewed as a favor given to a hobbyist simply because they see me frequently. Its a professional courtesy that I both expect from other ladies, and am happy to provide when asked to do so.

right after the appt before you leave.  You can ask her in your e-mail, "do you provide references for good/safe clients?" and that may help you decide to see her or not.  Or after the session before you leave.  

I also suggest that if she does NOT offer references and you have seen her: put that in the bottom of the review, "I asked if I could use her for a reference and she said NO, she doesn't give ref's"

the appointment.

The ladies seem a bit touchy about questions before hand.

Right as the session is winding down works for me.

As long as she gives references one visit should be enough.

If she needed a reference to see you, she should be equally willing to give one.  If she needed 2 to see you, she should be willing to give 2.

I am fine if a lady does not do referals, but she better have not bothered one of my lady friends getting one to see me.  If a lady wants referals, she better be willing to give them.  Otherwise, not a person I want to see, personal character and sense of fairness is more important than performance.  You can always find performance.