Checking the state of a checkbox in OS

Please clarify what you are asking about. I assume you mean checkbox in macOS.

There is .state, but I cannot find a way to check if the state is on or off. There is NSOnState, but it says it is deprecated, with no info about what replaces it.

When I use NSOnState in My Xcode 10.1, it shows me a message with clear info about what replaces it:

'NSOnState' is unavailable in Swift: Use NSControl.StateValue.on

    @IBOutlet weak var checkBox: NSButton!

    // Somewhere in an action method...
        if checkBox.state == NSControl.StateValue.on {
            //...
        }

Or you can use the dot-leaded notation of Swift:

        if checkBox.state == .on {
            //...
        }