How do I know if my supplements are legit?

I'm often skeptical about the authenticity of supplements on the market. How can I be sure that the supplements I'm taking are legitimate and not just filled with cheap fillers or harmful ingredients? Is there a reliable way to verify the quality and purity of my supplements before I take them? I want to ensure that I'm getting the most out of my investment and not wasting my money on something that won't work or could even be harmful to my health.