You know you’re doing the right thing when it feels effortless, provides a sense of accomplishment, and you receive some recognition. Also, you’ll notice that external stressors lessen. By whatever mechanism, it’s as if life is giving you more time and energy to focus on what you should be doing. On the other hand, life will rain down a shitstorm of external stressors if you’re heading down the wrong path. It’s like headlights and horns in your face as you drive down the wrong lane. If you don’t know what to do, do whatever’s easiest while eliciting the least stress. For instance, do predatory animals typically hunt the largest and strongest prey? No, they go after what’s easiest.
By effortless, I mean your task should suit your talent and tenacity. If you’re not initially good at something but have drive and perseverance, then yeah it might work out just fine. By accomplishment, I mean you should feel good about the tasks you perform on a daily basis and you should go to bed feeling satisfied. By recognition, I mean a few strangers should like what you do, compliment it or whatever. And finally, the last part might be magical or just a byproduct of intense focus, I’m not sure, but either way your surroundings should blur and intrude less in your life. When these 4 criteria align, you’ve done it, you’ve found your purpose. But bear in mind, this whole process might take decades, so have patience and try to enjoy the exploration phase.