I don't think there's anything that could make you feel completely sure about your aromantic identity. However, just like hemogoblin said, your identity is not diagnoseable. If you think you're aro, then that's it - you're aro. And no one has the right to doubt that. Obviosuly self doubt is normal, but that's why it's important to remember that labels are meant to help you. It's not like you're obligated to identify as something! Also being aromantic doesn't always mean being romance repulsed. There are romance favorable or indifferent aros, and that's okay! Aromanticism is a spectrum, so not all aros are the same.
You don't have to come out to her if you don't want to! But if her thinking you have a crush on that guy makes you uncomfortable, then I suggest you to tell her to stop prying. As your friend, she should accept this and stop.