If SpecialCells(xlCellTypeVisible) have already data then warn

i am using below code to add some data to SpecialCells(xlCellTypeVisible)

Public Sub macropastSC2()
Range("M1").Copy
Range("J4", Cells(Rows.Count, "J").End(xlUp)).SpecialCells(xlCellTypeVisible).Cells(1, 1).Select
Selection.PasteSpecial (xlPasteValues)
End Sub

i want to check first if above cell have already data then appear a msgbox otherwise run the code

thanx alot