Try the macro below. Just change the portion highlighted in red to reflect the first cell in column B to check.
Code:
Sub FindNumbers()
For Each vCell In Range("B1:B" & Cells(Rows.Count, "B").End(xlUp).Row).Cells
If Not IsEmpty(vCell) And IsNumeric(vCell) Then
If vCell.Offset(0,-1).Value = "" Then
vCell.Offset(0, -1).Value = "X"
End If
End If
Next vCell
End Sub
Regards,
Rollin