There are, of course, other ways (and maybe simpler) but I use a text box instead of a label and set the control source of the text box to something like the following:
=IIf(IsNull([cont_id])," ","Concentrator ID")
It checks the [cont_id] field for a value, if it is null then it sets the value of itself to a blank space (thus nothing shows) and if there is a value in the [cont_id] field, it sets itself to "Concentrator ID" which acts as the label.
Hope this helps.
Paul