I Just check out the ladies, look for a likely double act and conduct a proper interview making sure they understand what is required - ie two girls together, not one in bed and one hiding in the bathroom. All the better if they demonstrate a little girl on girl passion before clinching the deal. Last time the two girls almost snogged each others faces off before we left the bar, I love it when the girls show a bit of initiative in order to seal the deal.
Everyone has different requirements and differing ways of ensuring that they get what they want, the important thing is that everyone knows what is required before sealing the deal.