how to check overlap for dungeon ?

so currently i am trying to build a dungeon generation system . Currently i have set prefabs for each corner and straights. I have the map generating fine and it looks pretty cool. The problem im having is when that i have no detection to detect weather something has been placed in that location so use another prefab. I was thinking of tracking each one placed and then based on that, see what i can place next. i dont really know how to go about it.

         length = 30;
         GameObject current = GameObject.FindGameObjectWithTag("Start");
         int i = 0;
         bool first = false;
 
         while (i < length) {
             if (first == false) 
             {
                 GameObject instance = Instantiate (left_up [0]) as GameObject;
                 instance.transform.FindChild("spawn1").position = current.transform.FindChild("spawn1").position;
                 current = instance;
                 first = true;
             }else{
                 Debug.Log (current);
                 current = instantiateNext(current);
             }
             i++;
         }
     }
     GameObject instantiateNext (GameObject lastPrefab){
         List<string> placed = new List<string>();
         if (lastPrefab.tag == "upright") {
 
             GameObject[][] next = {right_up, horizontal_right ,right_down};
             int selected;
             
             GameObject instance = Instantiate (next[selected = Random.Range(0, (int)(next.Length))][Random.Range(0, next[selected].Length)]) as GameObject;
 
             instance.transform.FindChild("spawn1").position = lastPrefab.transform.FindChild("spawn1").FindChild("spawn2").position;
 
             placed.Add(instance.tag);
             return(instance);    
         }

this is just a little bit. but it pretty much duplicates down for each prefab.