Check if a certain gameobject is inside a collider

I want to check if a game object with the tag cube is inside a circle collider which is around the player. I want to use this for spawning cubes, but I only want to spawn cubes when the player is not surrounded by one.

 

Code (CSharp):

1. void OnTriggerEnter(Collider collider) {
2.    if(collider.gameObject.CompareTag("cube")) {
3.       //Do something.
4.    }
5. }

https://docs.unity3d.com/ScriptReference/Collider.OnTriggerEnter.html
https://docs.unity3d.com/ScriptReference/Component.CompareTag.html

 

I already thought of this one but that don't works for my special case, however I came up with a solution that works for me, but sadly only for my game