A natal chart won't be able to indicate if you're a twin flame or not and I also don't believe a synastry chart can either. Also keep in mind that soulmates(I know we have multiple soul mates some of whom aren't romantic) and twin flames are often conflated when they're quite different
A twin flame journey is a very personal one, very spiritual, and you'll have to follow your intuition. I do think if you look at a synastry chart between you and your twin flame, you'll find many connection markers, but they may not vary highly between those of a romantic soulmate connection. As far as I know and have seen