How do I know how secure is my iMac

Apple has taken exorbitant steps to build security into the operating system. It stays that way unless you give someone else access to your Mac, or your admin password. So no pop-up on your Mac advising that it found xx infections is legitimate, and Apple will not send you an email or contact your by phone to say the same thing. All bogus scams.

Apple Platform Security - Apple Support

macOS - Security - Apple

The recommendation that you will see repeated here for years on end is to not install any other so-called anti-virus or cleaner utilities on your Mac. Depending on your download habits, you may or may not install a third-party package that has other forms of malware attached to it that misbehaves when you grant it your admin password to install. For general forms of malware and adware, we do recommend installing the free version of Malwarebytes which will detect and remove known malware from your Mac. Malwarebytes can then be removed from your Mac if you choose via its Help menu.

Ways to avoid malware and harmful apps on Mac - Apple Support

Ways to keep your information safe on Mac - Apple Support

First, there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac.  This user tip describes what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community.  

There are no known viruses, i.e. self propagating, for Macs.  There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.   

Anti Virus developers try to group all types as viruses into their ad campaigns of fear.  They do a poor job of the detecting and isolating the adware and malware.  Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.

There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is designed solely to seek out adware and known malware and remove it.  The free version is more than adequate for most users.  

MacKeeper is considered by many users here to be akin to malware. I strongly recommend uninstalling it according to the developer's instructions. After the removal you can check to see if you've removed all of the supporting files by downloading and running the shareware app Find Any File to search for any files with the application's or the developer's name in the file name.  For subject software you'd do the following search(es): 

1 - Name contains mackeeper

Any files that are found can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.

FAF can search areas that Spotlight can't like invisible folders, system folders and packages.  

Other than that make sure you are behind a counter that uses WPA2Personal wireless security and that you use a strong login password of at least 8-10 characters and at least one capital letter, one number and one special character.

Why not advise users to keep their OS up to date? Exploits do exist, albeit rare, they don’t all depend on fault of the user. And the best defence is to keep MacOS up to date.

See information in Set up your Mac to be secure - Apple Support

and

Protect your Mac from malware - Apple Support

and

Security features for 24-inch iMac - Apple Support

often they get users to "hack" themselves

Phishing - Wikipedia

Recognize and avoid phishing messages, phony support calls, and other scams - Apple Support

Identify legitimate emails from the App Store or iTunes Store - Apple Support

Nothing can be known absolutely ... 100%. At some point you just have to trust the developers and use good common sense with respect to your internet activities.

And have an escape plan/ disaster recovery plan/ backups a plenty just in case.