You can ask the girl before going into her room (or before booking) if she does whatever service you desire. Especially if it's a less common fetish.
In this hobby like outside of it, a lot of the communication is done through body language. With experience, you'll be able to assess your partner's boundaries and needs. Sometimes you'll assess thing wrongly. That's why it's important you take slow steps towards what you want to do to them, making sure that your intentions are extremely clear. This way you give them plenty of time to stop you if they don't desire whatever you want to do to them.
In your particular case, start kissing her neck, then slowly move downwards, spend some time kissing and licking her boobs, then again slowly go down to the belly, and then go past her vagina and kiss her on her inner thighs. By now she will know exactly what you're going for and she will either cover up her vagina with her hand or let you go at it.
An important note since you're new to sex, always be aware of the risks of whatever sexual thing you do. Here's an article on oral sex and sexual transmitted infections:
http://www.ashasexualhealth.org/stdsstis/oral-sex-stis/
Good luck, hope you fulfill your greatest desires!