esams12, Pre-cum is a fluid that cleans your urethra and lubricates it. It cannot be stopped, just like your gf can't stop her vagina from getting wet when she is aroused. It does not have any taste or color and it is not related to ejaculate/semen at all.
She is asking you to tell her before you orgasm and ejaculate. Your semen can have a bad taste and she can gag on it. She might not even want you to ejaculate in her mouth.
If you cannot figure out when you ejaculate, you need to masturbate and learn what it feels like just before you have an orgasm.