I am one of the lucky ones. I knew that my parents were openly supportive of the LGBT community ahead of time, which made coming out slightly easier. However, if you are not sure, you can try to find out by casually bringing up the subject of LGBT rights and seeing how they react. If they have a supportive attitude to the community, then there is a high chance that they will support you when you come out. Even knowing that they will react well, it is still terrifying, but you will feel better in the end, trust me. In my experience, the first few times are terrifying, but it gets easier over time. I am now at the point where I openly talk about my sexuality and will refer to my crushes using "she" pronouns without hesitation.
Good luck on coming out, I hope it goes well