When you reach orgasm.

from a girls point of view: Everyone experiences it different, but for me every time I reach that point, I feel like waves of pleasure are sweeping though my entire body. Again and again. I don't feel anything but the pleasure, lose every connection to the world around me.black dots when I open my eyes.everything is only a blur and I am not capable of any coherent thought. Lost in bliss and a state of ecstasy.

from a med students point of view: you define it as a state of pleasure controlled by the nervous system. The muscles of your lower pelvis that surround you sex organs and your anus will start contracting and you will sometimes experience muscular spasms in other regions of your body as well.
Sometimes a clear liquid is secreted during female ejaculation. This does not happen every time you have an orgasm, so you cannot take this as an indicator whether you had an orgasm or not. There are other hints like your vaginal walls contracting/clenching.