how to check if my Mac has been hacked

If appear to be struggling to determine whether your Mac has been hacked, when you have full and complete physical and program access to it.

ANY message that you see in email or on the Internet, that tells you your Mac is infected with something is a Blatant LIE. No one can tell that from outside your computer.

If you are running Sierra, restart your computer to remove prevalent adware and malware automatically.

If that does not fix it, malwarebytes is a trusted application for removing most Mac-specific malware. There is no need install it or to have it running at all times -- you can use it as a Once-and-Done.

What makes you think it has?

You can download and run Malwarebytes:

https://www.malwarebytes.com/mwb-download/

You can also run an Etrecheck report and post the results here:

https://etrecheck.com/

That was a hack in Windows software that had not been updated with the patch Microsoft provided to counteract that hack.

It did not affect Macs at all.