How can I tell if I've been hacked?

I'm working on a 2013 Macbook Pro, running High Sierra 10.13.

I'm concerned that my computer is being remotely monitored, based on someone (an ex partner) from my past knowing intimate details about personal life, which I hadn't attributed to his accessing my computer until recently.

Most recently, I posted a vague, anonymous post on a very popular website under a fake user name, connected in no way to me. This ex partner somehow found the post and started interacting with it. I know it was him based on details provided.

I've run a Sophos anti-virus and Malwarebytes search, but they have all come back clean.

Is it possible that they've missed some kind of program that I can check for, or am I just paranoid at this point.