If i go to a spa and ask the mama sama to see the girls before I pick one.. would they do that?

Some do, some don't. You should educate yourself enough to ask for someone by name, but they will probably say "She busy" or "No one here by that name." If nothing else, just ask who is on duty and they may tell you and you just pick a name that is satisfactory to you....but don't be surprised if who you pick might have a different name later that day, or while with another hobbyist. The best way to be assured of someone that can take care of you is to build up some kind of professional relationship with a particular lady at each place that you end up visiting.

Excellent advice. Courtesy goes a long way.

A primo relationship with mamasan is a good way to get who you ask for.