Could remove everyhair on your body before she get's her hands on it, or just explain that you enjoy smoking weed you're not addicted and it's better than rampaging around drunk getting into fights, It's best to just to explain it to her in a quiet and respectful way so she understands you can use it responsibly and you don't take harder drugs.
Unless she is one of those uptight ultra right wing cristian nutters then cheat the test.