Sariel is right magic isn't some secret thing that awakens in you one day [meaning like with Sabrina when she turns 16 poof she's able to levitate and is a witch] but that being said some people wake up one day and will notice things differently, like they feel a calling.
i was always able to see/hear/sense spirits, and i had prophetic dreams, normal things in my family so whatever, never though i was a witch. one day in high school i had to choose my topic for a big end of year assignment in my world religions class. there was over 100 religions on this handout, but Wicca really stood out to me, so i choose it. a few months later i choose it was right for me. that's how i 'discovered' i was a witch.