Trying to check area for game objects that collide with spawnpoints, then spawn an item where ever there is not a collision

Currently i have an area i have designated for trees to spawn, with an array of spawn points for the trees to all be spawned on when the game starts, i need to write something so that i can get an array of all the spawn points where trees are not there(due to a player destroying a tree). I have tried working with Physics.OverlapSphere but to no avail. Any help or advice would be greatly appreciated.

Regards, Pdkkid