Array elements and making sure only 1 exists?

Ok Here goes.

This draws the corresponding image to screen dependant on what it stored in the array.

Function DrawFactoryMap()
	For Local yiter:Int = 0 To MapHeight - 1
		For Local xiter:Int = 0 To MapWidth - 1
			If FactoryMap[xiter,yiter] = 0
				DrawImage BlackTile,xiter*32,yiter*32,0
			End If
			If FactoryMap[xiter,yiter] = 1
				For Local r:RedTile = EachIn RedTileList
					DrawImage r.image, xiter*32, yiter*32,0
				Next
			End If
			If FactoryMap[xiter , yiter] = 50
				DrawImage g, xiter*32, yiter*32, 0
				GCount = 1
			EndIf
		Next
	Next
End Function

How do i check if Just one entry in to the array is missing?

For instance, if I loop through the array and do this:

if Map[xiter,yiter] <> blah
print Yay

it will print yay anyway because it loops through the array and finds elements not equal to blah.
I want to check if it's been overwritten and if it has been over written to set a global var to 1