When a monster is about to spawn, the Safe Zone app shows you a notification, and you'll also see a status effect. The notification can be hidden and you can show the effect again by pressing the key.

Monsters can spawn in many different areas, but the most dangerous ones are the ones that can be spawned in the base. To prevent them from spawning, use the workbench or the bonfire.

You can also use the Safe Zone app to show the area where monsters can spawn. This can be useful if you're trying to ensure that there is nowhere in your base that monsters can spawn.