If your indecisive on what you want to be when your an adult how is the best way for you to really know?

In addition to the advice already given, once you have identified some potential career paths, seek out some people who have those careers and talk to them to find out how they got there, what their job entails, what skills are needed, what the career prospects are (how easy/difficult is it to get a job in that field). As you narrow down your career options even more, you can consider shadowing people with those careers and volunteering or applying for internships in those fields.

And always keep in mind that once you choose a career path, you aren't necessarily bound to it for the rest of your life! You can always choose a career and then pivot to a similar (or even completely different) one later in your life. I started out as a doctor and pivoted to become a software engineer :)

I suggest that you think about what qualities you personally excel at, such as public speaking, giving advice to others or analytical skills. Then I would try to choose a career path that accentuates these skills.

You've been given some excellent advice already. One additional thought to consider is to gain insight into your true strengths and then look for opportunities to utilize those strengths in your education and career choices. My favorite assessment is StrengthsFinder. The resulting report will provide you with the strengths that you naturally possess. And it will provide insights into what opportunities may be potential options for you. This information may be very helpful in your decision-making process.