No, no, don't touch her anus. Most women will freak out. It is a sure fire way to kill an orgasm.
Also, it is not true that 'The only person that knows is her.' When a woman has an orgasm it is undeniable. All this talk about her faking it is just that fake.
Give this a try, slide a finger inside of her vagina. During the orgasm you will feel a rhythmic squeezing. Strong at first then less and less.
Or try this, have her masturbate, hold her close, gently touch the outside of her vagina. As the intensity increases, slip your middle finger about 1/6 of the way in. Just half way to the first joint. During the orgasm her vagina will pulse, this causes a small vacuum effect, you will feel your finger being sucked into her. With out a doubt you will know.
Also, notice how she masturbates, what she does, how does she move her hand, have her place your hand in the right place, then you take over, rub her the same way. Together you will learn.