How do I know if my iMac has been hacked?

Hackers are generally proud of their "work". Have you experienced any hacking messages, strange symbols or images, insulting or threatening messages and so forth?

Don't confuse a website telling you you've been hacked vs. actually being hacked. That sort of thing is scamming and phishing.

One common scam is spam e-mail telling you that your computer is infested by some huge number of viruses and that you need to do something right now (like buying a supposed anti-virus product) to fix it.

How would they know? It's scammers, trying to rip you off directly, get information that (or their criminal buddies) can use for identity fraud, or even hoping to connect to your machine and infest it with the malware!

The question is what makes you think your computer has been hacked. Please provide more details.