Paste into a Swift playground on macOS to see if your screen is locked

IsScreenLocked.swift

import Cocoa
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

var isScreenLocked: Bool {
    guard let wssProperties = CGSessionCopyCurrentDictionary() as? [String : Any], (wssProperties["CGSSessionScreenIsLocked"] != nil) else {
        return false
    }
    return true
}

Timer.scheduledTimer(withTimeInterval: 5.0, repeats: true) { _ in
    print(isScreenLocked)
}

Shows how to use the CGSSessionScreenIsLocked attribute to know if the screen on a Mac is locked at that time. You can start this running, lock your screen, then unlock and check to see the results.

This code is using a simple timer, but you could just listen for kCGNotifyGUIConsoleSessionChanged to be posted and then check to see if it is locked.