Check if position is free.

Hey there, again I'm stuck and I can't seem to find a solution.

I already checked alot of examples and forum posts :)

So, I can place and destroy blocks just fine in my game.(using create object at current mouse.x and mouse.y)

The only problem is: I can place 2 and more of the same blocks on the same place. So is there any way how I can check if there's not already a block at the current Mouse.X, Mouse.Y?

drive.google.com/file/d/0B-5Zgs_yQiBiM2xBZEpRcHo4NDQ/edit