By observing your child and listening to what your child says. Don’t tell them what you want them to be, but listen to what they say they want to be. Their interests may change over the years and that is okay. They aren’t supposed to choose a career at age five. They need to explore different possibilities in a safe environment.
So supply them with books, classes, and the tools to explore those interests, and stay vested in those interests. Go to their games and productions. Take them to museums, movies, and libraries. Be involved. Talk to them. Listen to them. Read to them. And start at a very young age.