Can I check if someone is using my Wi-Fi?

Excuse me, I was wondering if you could help me with a quick question. I'm a bit concerned about my Wi-Fi security and I was wondering if there's a way to check if someone else might be using my network without my knowledge? I'd like to ensure that my connection is secure and that no unauthorized users are accessing it. Is there a specific tool or method that you recommend for doing this? Any help would be greatly appreciated.