This question can mean a lot of things and be taken in many different ways. It also ways heavy to think about. Well the first thing that someone may ask is, How do you know what? I would say think about the question. What would come to your mind first if it was asked to you. For me it would be, How do you know...everything will be okay and life works out? That is something that would first pop into my mind. This is simply because I have anxiety. I think all the time about the what ifs, how to, and what could be. Every possible worry and outcome whether good or bad is always there in my mind. Therefore that type of question would always be there for almost anything actually. Someone with anxiety will always ask, How do you know? Because the reality is we do not know. The future is not predicted. We can only live for what we feel and what we want to live for. Because allowing those heavy questions to weigh down our lives can make or break someone. I am still learning how to turn my thoughts off or at least control them when they are too loud. Worrying can turn to negativity and that definitely makes life harder. Figure out when to turn the negativity to positivity.
Another way to think of that question is How do you know...they are the one? That is a big one in life. That is something only you, yourself can answer. Nobody else can tell you how you feel. You can ask other people how did they know. Everyone "knows" by different experiences. I believe it is a combination of how you feel and experiences that have made you the person that you are. I say this because not all people are in tune with emotions. And past experiences make who a person turns into. Two people together figure out if they connect on that level or not. But again that is a deep question.
How do you know...when to quit a job or what your dream job is. How do you know what your passion really is? How do you know when to walk away? How do you know that you have a mental illness and not just a little different from people? How do you know if your child has ADHD? There are so many dimensions in life. This is why there are so many decisions to make in life. But life is never worth giving up. And there are always second and third chances. You can always figure out what you want and keep learning along the way. Because we are not here to just do one thing and maybe realize we got bored of a job after 10 years, become depressed but feel stuck. Don't let that happen. Keep moving forward!